Well first of all, having characters usually means quite a lot of polygons and textures, I would first check the poly count of those characters you are using and check the sizes of textures and so on.
And if those characters are animated using skinning/skeletons, then you have skinned meshes…
And 120 of those sounds like quite a lot for anything. You could make a crowd out of those.
“What is the maximum animations or complexity for the quest?”
Maybe just do an artificial test scene and see which one causes the slowdown, polys or skinning or amount of both. There probably isn’t any specific limits you can define like you asked, except hardware limits.
You could then decide to remove detail, animation and add LODs for both models and animation and so on, IMHO it isn’t very specific to any platform.
If your animations are simple enough (slightly bending/twisting, slight arm movement etc.) then just encode motion influences into vertex colors and write a vertex displacement shader to animate them all. You can’t have 120 skinned characters on the quest, skinned mesh renderers don’t get dynamically batched nor gpu batched even if they’re the same mesh.