He's exaggerating, but having thousands of partners isn't unlikely for faggots. I mean, the main thing that stops most men from having sex all the time, is the fact that women are far less willing to have sex than men are. That problem disappears if you're a faggot. Imagine every one of your potential sex partners having the same sex drive as you, no inhibitions, no standards, while also being a sick fucking pervert on top of it.
Unless you're grotesque, you can just go to any gay club and have sex, probably with more than one guy.
The main thing that implies that he's lying through his teeth is the fact that he didn't have every single STD in the world. Gays are riddled with them, and if he had slept with 10k men, then he would have gotten some kind of disease, no matter how much he tried to protect himself. It's not unlikely to be in the thousands though.