If I had to give a ballpark figure for fun, I'd guess that 60-70% of professional athletes dope.

There was an anonymous test, I think it was done a while ago by the Athletics federation. 40% admitted to taking some banned substances in the last year or something like that. We can assume that it's an even higher threshold given the incentives and how cutthroat is the competition between elite athletes.

Edit: lol it was actually 57%

https://www.theguardian.com/sport/2017/aug/29/sport-doping-study-revealing-wider-usage-published-after-scandalous-delay