Right now we have 22 snakes, 1 lizard, 2 cats and 2 dogs. I know I'm at my limit mammal-wise, but snakes...yeah I just dunno. There are always new morphs that I see that I want so I've had to teach myself to focus on the projects I currently have on the table...
I feel that when it turns into a chore, you don't enjoy it, or can't properly care for all of them, it's time to consider downsizing your collection.
I, personally, feel comfortable where I'm at, but I do have the time and space for more. I think if your collection grows slowly it doesn't seem as overwhelming as if you added 6 snakes to your smallish collection in 1 day.