I think it depends on many factors like size of the animals, if Im breeding, obviously money and space and time.
My personal limit as a pet owner is around 6-8, time to spend with each one individually.And low enough numbers to provide big habitats.
If Im breeding(my corns for example) then I would have room for more because they would be in racks.Babies are only around till sold so habitat size isnt an issue nor is taming them down.
Its easy to get caught up in the whole "collection" thing and end up with too many.