The best I ever did was 36, so I put that, although now it's only about 20 (Blame it on the knackered shoulder - I'm out of practise). I bet half the people will lie and say they can do "about" 50 when they've never tried. It's a lot harder than you think.
If you're doing say, 30, the first 20 or so will feel really easy, and then by the time you get to 30 your arms will be dead...
Chin-ups are the man's game though :-