In D&D 5e at level 12 a wizard can have up to a maximum of one level 6 spell slot and two known level 6 spells in terms of the highest level stuff you can cast.
Overall you will have a grand total of 28 known spells and 5 cantrips between spell levels 1-6.
As for NPCs that might go above player levels and cast higher level spells we don't know.
Last edited by Darth_Trethon; 29/07/23 10:42 AM.
