If we are looking at all the countries combined, I would say that it is probably between 10 and 20 percent of athletes that have committed doping violations in the Olympics. This is pretty high, but not horrible. I would say that most of the athletes do not use performance enhancing drugs.
I would suspect that over half of the athletes in the Olympics have committed doping violations at some point in their training leading up to the Olympics. The Olympics is a once in a lifetime chance for most athletes. The incentive for doping is way to high for them to not give it a try.
I think that doping violations go on at the Olympics all the time and athletes don't get caught. I think this has been the case for at least 20 years now. There are some who get caught during the Olympics or after but in general most get away with it.
There are many cases of athletes being banned for doping whether it happens at the time or years later. There are also many cases of the athletes getting away with doping and never getting caught. It is an unfortunate part of the Olympics and likely something that will never change.
In the past few decades, many athletes have been caught doping while preparing for or competing in the Olympics. Athletes in all professional sports, including the Olympics, try to cheat the system from time to time. A lot of these cheaters get caught and punished for doping, but others get away with cheating for a long time.
We'll likely never actually know the answer to this question. There will always be athletes who are doping who haven't been caught yet. As technology increases, both the ways to cheat and the ways to catch cheaters will get better, so even athletes who were "clean" for many years may be found to have been doping.
Athletes that are able to compete in the Olympics are usually at the very top of their respective sport, and have taken great care of pride in getting there. They have nothing to gain and everything to lose by doping to get ahead in the Olympic Games. They also know the strictly enforced doping laws, and even if they did decide to take a performance enhancing drug, I doubt they would be able to get away with it for long.