Right now, I'm at 13. Plus 2 or 3 "keepers", and, if I ever get around to selling her, minus one Lucy. I try to stay away from the bigger stuff so I have more space/money for the smaller stuff. Boas are very hard to resist, though. =)
I try not to set limits (too easy to surpass, and too hard to obey!), but I'd tentatively say 20 until I'm out of college and have a stable job... I'm not sure how well that's going to go, though, since my mom is trying to convince me I need a couple Dumeril's boas and there's a few more things I want before I stop.
I'd have set my limit a lot lower, but (aside from getting carried away) I got lucky--my parents have embraced the scaly things, and then some! My dad has claimed the beardie as his, and my mom is about $200 and a nice enclosure away from becoming a fellow hobbyist/addict by way of a Dumeril's boa (which fits with Dad's desire for a large snake of his very own).