It just takes finding your own personal limit to how much you feel like you can take care of, without it becoming a burden and causing you to burn out on it. I've done 125~ breeding animals and 700~ babies in a year before...I don't want that again. 75 breeders and 180-200 babies is much easier to me (different species now, a few boa and python species, no colubrids) although I was attempting to stay in the 60~ range of adults to be safe. Finding an awesome girlfriend that shares the hobby makes a huge difference and the 75/200 could easily go up by about 50% and still feel quite manageable.
I will say this though, what begins to get to me first isn't the reptile care, it's the rodent breeding, which I much prefer over buying them...and the girlfriend is amazing at helping with that too.
It's all about finding personal limits though and keeping yourself under them before it starts to effect either you, or the care your animals get.