Around 3 or 4 different monsters - each with different behaviors and attacks is usually plenty challenging
We have so much variety with beasties now too
Emera,
Skel Commnader is a good monster to test custom brain work with!
Working with the brains is quite fun - check out:
1.the asset pack monster definitions
2.script entities within the main game ( u open LoG2 up in the editor )
3.brouse through the functions for Monster and Brain components
Im anticipating some stella custom works from the community with custom monsters,
keep your eyes peeled + good luck!