I dont think there is a definitive answer for that, its more the less the better without sacrificing visual quality. So if you have a larger model, it can have one texture set and if you have a bunch of tiny models you should create one texture atlas for them instead of giving each model an individual texture set.
Ideally models should have consistent texel density based on how far from the player they are. Objects that are viewed really up close need more texel density than objects in the background. Check out this for more detail: https://www.artstation.com/artwork/qbOqP
