You can be a multiclassed character with all classes at level 12, but only with point buy
Point buy can leave you at 13 in all stats, except CON, regardless of race.
Let's first make an overview of the stats needed for multiclassing:
Class Ability - Score Minimum
- Barbarian - Strength 13
- Bard - Charisma 13
- Cleric - Wisdom 13
- Druid - Wisdom 13
- Fighter - Strength 13 or Dexterity 13
- Monk - Dexterity 13 and Wisdom 13
- Paladin - Strength 13 and Charisma 13
- Ranger - Dexterity 13 and Wisdom 13
- Rogue - Dexterity 13
- Sorcerer - Charisma 13
- Warlock - Charisma 13
- Wizard - Intelligence 13
To summarize the stats on 13 needed:
- 3x STR
- 3x DEX
- 0x CON
- 1x INT
- 4x WIS
- 4x CHA
So we see that CON is not used, we can keep that low (which also means low HP)
Let's also take in account that there are restrictions on the first class you pick, according the PHB page 163 (Thank you Theik):
To qualify for a new class, you must meet the ability score
prerequisites for both your current class and your new one, as shown
in the Multiclassing Prerequisites table.
Races give a way to compensate or raise the stats if needed.
Dwarf - CON+2
- Hill Dwarf - WIS+1
- Mountain Dwarf - STR+2
Elf - DEX+2
- High Elf - INT+1
- Wood Elf - WIS+1
Halfling - DEX+2
- Lightfoot Halfling - CHA+1
- Stout Halfling - CON+1
Human - All stats +1
Dragonborn - STR+2, CHA+1
Gnome - INT+2
- Forest Gnome - DEX+1
- Rock Gnome - CON+1
Half-Elf - CHA+2, 2 other stats +1
Half-Orc - STR+2, CON+1
Tiefling - INT+1, CHA+2
With standard array, we start with the following possible stats:
15, 14, 13, 12, 10, 8
Since there is no race in the PHB with a +3 to make the 10 a 13, this means we cannot get all stats to 13.
For standard array it is not possible to create a level 12 character with all classes.
With point buy, you can easily get all stats on 12, bump 1 stat to 14 or 3 stats to 13, and then pick Human for a +1 in all stats.
Or you can go 13 in all stats, but 10 in CON (or 8 and spend the leftover points elsewhere), and pick a race you want.
All-round character: good at everything
All the 1st-level bonusses of the classes (except all proficiencies, see PHB 164 for specifics)
All weapons and armor proficiencies (except Heavy armor, you'll only get this if you start with Fighter or Paladin, or pick certain Cleric domains)
A few more skill proficiencies: single proficiency from the bard, rogue and ranger classes (thank you Theik)
Access to higher level spell-slots
Lots of utility and flexability
Lots of Roleplaying opportunities
The disadvantage is that you're an all-round character: you're good (not great) at everything, but you don't excell at anything.
You have lots of utility that can be used in Roleplaying, but in combat you can only pick a limited amount of actions per turn.
Proficiencies gained can be redundant since you already have it from a different class.
Since the levels are spent on new classes, no ability score enhancement of feats can be taken.
Futhermore, you only get the 1st level skills and bonus of each class: this does not scale very well at higher levels.
No access to higher levels spells due to low class levels.
No access to extra attack.
No access for the sub-class of most classes (mostly requires 3 levels in the class)