Spellchuckers: When does their power "top out"?

[Klorox]

Just wondering. I'm thinking about my proposed party, and instead of certain casters going very high as casters only, I'm thinking about mixing in other levels just to give a more sturdy overall character. For instance, if you multiclass in a level of Monk, Cleric, or Bard, you're really helping your saving throws out a lot. Monk adds Evasion, which is great if you don't wear armor, Bard allows you to sing if you can't get into combat right away, and adding a level of Fighter or Ranger can really help out a character to become a much better combatant. A level of Barbarian adds a ton of HP and a Rage, which is nice too.

So, when do Clerics "top out"? What about Wizards? And Sorcerers?

TIA

[Aerich]

It depends what you mean by "top out". Spellcasters continue to gain spell slots up to lvl 30. If you consider a spellcaster to have topped out when it no longer gains 9th lvl spells, I believe it is around lvl 20 for all major spellcasting classes.

Mixing in other classes after lvl 20 will mean that you lose a few lower level spell slots, but considering that the party is likely to be in HoF at that point, it shouldn't be a big deal.