From game statistics 16.8% at one point touched ironman, around 13% stayed a bit longer and 4-5% actually played to any significant extent. Assuming info from other steam games holds true I would imagine the group of people who can be called players of EU4 is somewhere between 5% and 15%, 10% if I had to give a number.
It is worth keeping mind that from what I have seen the people who got the easier achievements then stopped playing ironman is bigger than those who never played with it at all so even the 5% there doesn't mean it is mainly made of ironman players.
EDIT: @Edmon ironman enforces absolutely nothing. A person can play within the supposed rules without it and a person can play outside them with it. Even without going into third party programs bugs, exploits, mistakes and unintended results pretty much guarantee that.
Circumventing the strict rules is called "Cheating". Ironman specifically states, in plain English, that it disables saving "So you can't reload when something goes wrong for your nation". If you then find a method to specifically do exactly that, you are a cheater. You've been told in English, which I'm sure you understand, to not save and reload when something goes wrong. The game should not have to (but does attempt to) enforce that rule, but you should have the morale fibre to follow the ruling without enforcement.
Exploits are different, because there is no statement somewhere at the start of the game that specifically and explicitly says "Thou shall not use the guarantee trick" for example.
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