Here we run into a charming double standard, which a great book summed up with its title: "He's a Stud, She's a Slut".
Being a manwhore generally isn't about quantity, although that's a factor. Generally, a manwhore is a guy who will stick it into anything if it'll have him. You, your sister, your mom, your aunt, your coworkers, the woman at the next table, HER sister, he doesn't care. Pretty much he has a total lack of standards; no matter the woman, he'll do her if she's willing, or drunk enough to think she is.
Oh, or he charges money for sex. That would also make him a manwhore.
Loving that you bring up the double standard. It is extremely frustrating!
My definition of a man whore: He's never met a vagina he didn't like.
These guys will screw anyone, and are also cheaters, all likely based around deep insecurity.
I had a man whore once.....smart guy. Master manipulator. I bought it the first time. Then told him I knew what he was up to. I never heard from him again. They don't like it when you are smarter than they are. : )