I think the problem with that is...who can really define a reasonable amount?
Example. I rescue but often end up with dogs. Luna's ex owner wanted her dog to go to a raw fed home. Loki was a present from the boyfriend after my last dog died. Spyro I have had since I was 10 and I have considered finalizing the adoption of Chewy, a friend's dog that they personally feel is getting too expensive. I have had 4 dogs before and had no problems. What if I inherit another dog? Does this make me a hoarder even though all of them are in good health, licensed, and happy? What about the neighborhood cat lady and her twenty something cats (also healthy.) What about ranch owners or breeding kennels/stables that often have well over 10 animals?
Reasonable amount is affected by two things.
2. ability to care for the animals
3. mental state
4. ability to give away pets, if need be
if you are able to care for 40 cats and it's legal for you to do so, and you mentally aren't a hoarder, then there is nothing wrong.
I suppose how you get the animals plays a part in it too. If you went out and brought 40 cats? I'd question your mental state. If you work at a kill shelter and can't stand to see animals put down, so you take home every cat that is healthy but reaching the end of it's adoption period, I'd see it differently.
also, lets say for some reason you can't look after them all, if you are able to give them away, then you aren't a hoarder. if you can't give them away, even if you've found good homes for them, then you're a hoarder.
There probably are other factors too