out of married couples?

I've read 30% and 60-70%. I don't know which one is more accurate. All I know is it increased after women entered the workplace. More women cheat now than before.

You can look up stats for various countries/cultures, but you'll find lots of different information based on when the study was done, how large the study group was, and what they considered "cheating" in the study.

Short answer: Too many.

I would say it correlates quite closely with the divorce percentage so around 70%

Depends on culture of the society. I am a Indian and less than 5% are cheaters.
of course it depends on what do you mean by cheating also.
Coming late for date and saying was in trafic jam or somthing like this if you have in mind then I would say 99.99%