I think in the end its when the keepers or animals health and well being is being placed at risk simply due to the mere existence of the "pets".

-A keeper maintains a certain number of animals based on their abilities to properly care and feed them animals with out putting themselves or their pets at risk.

-A hoarder keeps a number of animals far beyond their abilities to properly feed and care for. They may truly care for the animals but their pets have far exceeded their abilities to keep the animals while maintaining a healthy environment for themselves and the pets.