I don’t agree about bodybuilders being stupid, but going to the gym and being a bodybuilder are not one and the same. They just happen to occupy the same spaces.

And in bodybuilding, unless someone is specifically a natty/natural bodybuilding competitor the answer is pretty much “all of them have been on gear at some point in time”. For other gym-goers, the answer is, nobody knows unless they tell you and since steroids are very illegal, people don’t tend to shout it from the rooftops.

In the US, between 15% - 30% according to https://pmc.ncbi.nlm.nih.gov/articles/PMC1484557/

I would guess this number is higher today