I was building a Sorcerer character I had in mind for high-level campaigns (e.g. starting at 10th) and noticed that while I was planning and selecting his spells known, I seemed to gravitate mostly toward attack spells. Then I started thinking: I can pretty much use any one of these spells as many times as I want within the limits of my spell slots, would it really do me that much good to have this many of my spells be attacks? I've kinda made a point to grab at least one spell per spell level that can deal multiple damage types and/or at least one Magic Missile variant at appropriate spell levels, so should I bother with any more damaging spells with a selection like that or should I look for more utility/support spells, instead?
Common wisdom says that direct-damage blasting is a suboptimal route for high-tier casters. Damage that doesn’t kill leaves an enemy at full capacity for hurting you or your allies, plenty of low-tier classes can only deal damage, and tend to have an easier time dealing it than a caster does, most direct-damage spells are pretty poor, and so on. There are a lot of reasons to avoid blasting altogether as a caster.
However, it’s worth noting that, at the high end, sorcerer blasting is pretty terrifying and extremely reliable, even by the standards of high-tier, high-optimization builds (see the Mailman). Immunity to HP damage or death thereby is extremely rare, after all, and immunity to being dead just doesn’t exist. And the sorcerer is well-placed to do it really well, what with access to the Sor/Wiz list plus arcane spellsurge. At this point, you’re not handing out damage, you’re handing out death.
So good sorcerer blasting is very all-or-nothing: either you kill, or your blasting was a bad idea in the first place. Much better to apply some very-debilitating condition than to deal a bunch of damage that won’t actually kill. Even if an ally mops up the remainder before the enemy gets to go, that’s an entire turn the ally didn’t spend dealing with a different threat.
Now, if you do want to go the blasting route, do you want to know a bunch of attack spells? No. Knowing any at all basically means you’re “specializing” in it, relative to most sorcerers. You build your blasting on one or two key, excellent spells, not on knowing a bunch of different blasts. That would be pointless and redundant.
Because that is something that is always true of every sorcerer: you do not want redundant spells. You get an extremely limited number of spells known. Your total spells known is, in fact, very similar to the spells a wizard gets to prepare every day, but where he’s only got to worry about his spells being useful for the next 24 hours or so, you have to make sure they’re pulling their weight for the rest of your life.
So no, under no circumstances do you want a “lot” of blasting spells. You may want a “few” blasting spells (like literally 1-3), but even if you specialize in blasting, you still shouldn’t know too many direct-damage spells; a few well-chosen spells should be quite sufficient. You need the remaining spells known to ensure your mobility, battlefield control, versatility, defenses, and so forth. And if you’ve chosen your blasting spells well, there should be no reason to use a different one.
Personally, I’d probably get some lesser orb at low level, swap it out at 8th or 6th (or the last even Sorcerer level I took), get a real orb at 8th, and only get another once I hit the orb’s Caster Level cap. That is, if I was actively trying to be a blaster. If not, I’d be fairly likely to not bother with any at all.
There's no requirement to know a lot of attack spells at all. But see below:
Blasting is actually suboptimal
Sorcerers get tons of mileage out of buffs (haste), debuffs (slow), summons, battlefield control options (solid fog, wall of iron) and those options are almost universally going to be more powerful both in and out of combat than pure damage. The only hit point that ever matters is the last one, you know? Enemies are dangerous and active until they drop dead.
It's not how many you know, it's which ones you know
Simply dealing damage doesn't mean it's a good blasting spell even if you want to go that route. Look for spells that don't offer spell resistance and that either don't make an attack roll or make a touch attack. The various orb spells are blaster staples for a reason.
We Built This Necropolis on Metamagic
Optimized blasters based on Sorcerer use three things in conjunction with each other - metamagic feats, SR: No spells, and Arcane Spellsurge. Spellsurge lets you sling two spells with the same action, and metamagic turns an ordinary spell into an extraordinary one. Easy Metamagic, Practical Metamagic, and Arcane Thesis start you on your journey, which you then improve typically with Incantatrix levels and access to the Invisible Spell feat to jack the overall 'price' down. Sorcerers lay down the law with maximized, empowered, twinned/split and/or chained spells that end up dealing reams of damage to many enemies at once - enemies that are powerless to resist that damage. Energy admixture or energy substitution lets you work your way around energy resistance, or you can go for a feat like Searing Spell and just not care. Supplementary spells like surge of fortune (to auto-crit) or wings of cover (for defense) mean you can afford to go balls-to-the-walls when the situation calls for it.
I am the action economy
Going further, quickened spells and contingencies mean you can crack off more blasts per turn, or go Full Nova Mode if you need to. Between Quicken Spell and Arcane Spellsurge you can potentially fire off four blasts on one turn and not even care about the amount of spells you spent to do so. The Craft Contingent Spell feat lets you store up blasts that you can crack off as part of a free action (via command word, for example), letting you cast blasts or other spells as non-actions.
Just remember this:
Even with the above, you're better off not blasting.
No. Yes, somehow sorcerers have traditionally acquired the reputation on focusing primarily on damage spells, but there's no inherent reason why this should be so. You can just as easily have a sorcerer who focuses on buffing, battlefield control, or utility spells.
And like you say, you can spontaneously cast any spell you know, so even if you know only a single attack spell, you can always select that one. Spending more than one spell known per level on damage spells seems like a waste to me. Take (for example) just Magic Missile, Flaming Sphere and Fireball, and fill out the rest with stuff like Grease, See Invisible and Haste.
Or even just Magic Missile; it's a great, versatile attack spell that scales with your level. And there are lots of other ways spellcasters can contribute to combat than just doing damage. I mean, anyone can do damage, but only you can redefine the battlefield.
Compare the needs of a sorcerer to a swordfighter and you will quickly realize that "doing damage" is only a tiny part of what you need to know, it is in fact pretty simple: "stab them with the pointy end".
Most of the things you need to learn involve parrying / getting out of harm's way (defensive spells), how to break somebody else's guard (control spells), etc.
Knowing seventy flashy ways of putting the blade into your opponent is not going to result in much if they parry your first attempt to do so and prompty seperate your head from your body. This counts doubly-so for sorcerers who:
Only if blasting is your specialty, and even then, you need to be careful.
Sorcerers do not specialize in schools of magic, the way wizards can. But the number of spells they can know is very limited, and this forces them to "specialize" in their own way: it just cuts along very different lines. There are no rules for this: just pick a theme and run with it.
You can choose to specialize in dealing damage, if you want. Many PCs do this, and rare is the NPC army that wouldn't want to have a blaster-specialist on its side. But there are many other themes that can be invaluable to a party: buffer-specialists that specialize in making the party harder/better/faster/stronger, controllers who use things like wall of (whatever) to cover their allies and force the enemies into uncomfortable positions, logisticians who get the party into (and out of) places and situations they couldn't ordinarily go, and so on. Any non-blaster specialist can also be especially good at the ever-popular player pastime of making the DM cry: every DM knows how to handle a blaster, but they usually aren't as used to dealing with other themes.
Even if you do not specialize in blasting, it is generally a good idea to have a couple of damage-dealers as a last resort. Sometimes the enemy will break through your defensive lines, and you will want to have a backup to (hopefully) keep you alive. But unless you're actually specializing in blasting, you don't want to waste very many spells on it.
Not at all.
While it's pretty typical for people to do so, heaping up attack spells is actually a pretty bad choice, since A, you only get a handful of spell, B, you're pretty much stuck with them aside from being able to trade 1 out that's at most 1 level lower then your highest castable level when leveling up, C, it's REALLY easy to counter blaster sorcerers.
When I roll my sorcerer, I pick my spells based on what I would literally want to have as a part of my daily life. Unless your sorcerer was raised in a militant culture and was trained from birth to be a magical heavy assault weapon, more often then not, damage dealing spells will have little to no place in your spell list. A lot of people also forget that you need a fair amount of down-time to recover, and getting the maximum efficiency possible from your slots remains just as important to a sorcerer as to a wizard. Being a sorcerer just removes the headache of trying to fore-see which spells you'll mostly likely need memorized, but at the same time requires that you be able to cover your bases. Sorcerers are typically loners, and that does imply that you need at least some degree of self-sufficiency in their spell selection.