It really depends on how quickly you cycle the yeast, and how healthy you can keep it. I've heard 10 as a rule of thumb, but I call BS on that. I've heard of commercial brewers repitching to 500+ batches, but I've also heard of homebrewers getting funny flavors after 5.
There's what @sneezles61 said, which I coincidently just learned about at Homebrew con, brew a larger starter and reserve some of that. Also, If you're going to establish a "house strain" where you use the same yeast for almost every beer, you can probably go further than if you use a strain every few months, and leave the mason jar dormant, taking space in the fridge in the between times.
in other words, you'll have to see for yourself because my number is very likely not your number.