How common is it for celebrities to sleep their way to the top?

This is a difficult question to answer. A lot like asking how many master spies are currently in country xyz.

We might gain some insight from how many people fail to do so when trying. The majority probably fail because they're not celebrity material and being a prostitute isn't really a very good way to earn respect.

Of course just because they manage to get to the top doesn't mean that they can hold that position. 95% of their career its being interesting to the public. Likewise, if they are interesting, they probably don't need to prostitute themselves to find their way up.