It varies a lot. The general is 7-8 spurts, some of which are really...high recoil ;P
My "record" of sorts must've been one time I had anal sex with a former girlfriend. I've asked her moments earlier where she wanted it and she had told me that she'd like it on her ass. I guess I lost control a little sooner than I wanted to and I spurted out a couple of times into her before I "regained consciousness". I tried to make up for it by pulling out and shooting the rest on her butt, and it was insane! I swear, I had like a dozen more spurts, a lot of these where after the orgasm in itself was over, so I just stood there and thought "what the hell is going on?"