In most fanfictions/headcanons Josh is that type of guy who sleeps with a lot of girls. What makes people think that?
I mean it's his general flirty behavior I think. Also the gross stuff he says about women in general (see how he talks about Ashley to Chris. Call me a prude but I don't think you should be talking about doing explicit acts to your best friend when you KNOW he likes her).
He just gives on very frat boy energy, and well... he's rich, good looking, and has a famous dad. So if he wanted to sleep with a lotta people (which by his promiscuous behavior it seems to be the case) then he can pretty easily.
I like to headcanon him and Jess as on and off again fuck buddies, cause I think it's fun, and I think the two of them are sensual people who would want no strings attached fun. Ironically Mike seems like he’d get a lotta chicks too but I get the vibes he flirts with a lotta chicks but doesn't really sleep with any of them unless he has some sort of emotional attachment to them (since he's secretly a grampa we all know this).