For a start. Most men will ALWAYS lie when they say how many people they have slept with if asked that question by another man, especially if they believe it is the norm for men to sleep with 'x number of people' just so they can't be mocked for having inexperience or being a virgin themselves.
Notches on the bed post are meaningless, even if your friends claims of having sex with 12 girls is true. That's 12 girls who didn't think he was worth staying with, 12 people who will remember him for sleeping around and not being faithful. That sort of reputation sticks for life in some social circles. Your own partners may have cheated on you, but staying a good and faithful person will be rewarded when your older when people around you start seeking long-time relationships.
Ignore his jibes, he's probably riddled with more insecurities than you are and is simply trying to project them on to you rather than face them himself.