If he says a definite number like 2, i think it would be correct. If he's vague, like he says 2 or 3, it's probably not true. He should remember each and every one! Unless he was totally incapable because he was drunk and wasn't sure if he did or not.
A vague number like 20 or 30 probably means he doesn't think much about girls after he's made his conquest.
I think the only times a guy would lie is if it's a huge number or none at all.