What are the rules for ability score maximums?

Question

There are a bunch of rules I seem to remember about ability score maximums, but I can't remember where they come from or if they're actually the rules as written:

• I believe there is a RAW that you can't exceed 18 on any of those either at creation or by buying points every four levels. For example I started with DEX 15, bought new points in DEX at levels 4th, 8th, 12th, and as for level 16th the cap of 18 will already be reached and I will have to improve another characteristic instead. Did not mention it but of course, racial bonuses if applicable can shift up the cap.

• I believe there is also a RAW that you can bypass the cap by using a maximum of 5 tomes, making the real cap at 23.

• Finally, for a standard human who bought 5 tomes and wears gloves of dexterity +6, the hard limit for its dexterity would be 29. (I purposely did not pick the example of strength because I believe there would be some way around it. Also in my universe, I have a way of getting one more dexterity point and reaching 30 but that is not the matter).

Are all those statements corrects, and (if so) where are they written?

Answer 1

No rules such as those you describe exist within D&D 3.5. There is no limit to how high any ability score can get.

I obviously can't know where you got the idea from, but here are some possibilities:

• An overeager interpretation of the point buy rules (Dungeon Master's Guide page 169), which do not allow a character to buy a base score of more than 18 at character creation. This has no impact on characters after they have been created.
• Inherent bonuses (Dungeon Master's Guide page 21) are limited to a +5 bonus total. This includes bonuses available from the Wish spell and from Tomes, but does not interact with bonuses gained upon level-ups, and does not imply a cap on final ability score numbers.
• Houserules from a gaming table you may have played at earlier.
• Rules from a different gaming system.

Answer 2

D&D 3.5 has only some caps on ability scores.

Before applying racial modifiers, your scores can be between 3 and 18 if you're rolling dice (standard method), between 8 and 18 if you're using a point buy or between 8 and 15 if you're using the elite array.
Since as I already said rolling dice is the default, and you roll 4d6 removing the worst one, you can see why 18 is used even for point buy.

As you correctly said, there's limits on how many bonuses you can get from enhancment items (max +6 pre-epic, max +12 epic) and from wishes or equivalent sources (max +5). However, the points spent during level-up, some epic feats, some prestige classes, some grafts, some spells have other means of raising your stats and those might be capped per source (like a "+2 every 4 levels, max +10" I just made up) but the total is never capped. You can reach as much as you can by stacking the resources you've gained through money and XP.
This is lampshaded by the infamous Book of Vile Darkness combo building up +2 Str every day, no upper limit. or Pun-Pun getting nigh-infinite of every ability score at level 5 (without deity intervention, otherwise it's level 1 and yes, this is theoretical optimization and you should totally never play these builds).

Answer 3

The only limits to ability scores are:

base: maximum of 18 (standard methods)

race: no official limit

enhancement: does not stack, so the highest you can get is +12 to one ability score

inherent: +5 gained from wishes and tomes