As posed, the OP's question isn't straightforward to answer. In part, it doesn't consider bias, or quantify what "know" means, in terms of casual acquaintance, colleague, friend, close friend or confidant, etc.
In my case, I can't readily, honestly assess how many female colleagues or casual acquaintances have engaged in extramarital sex. A few of them I suspect have that tendency, but I've no real certain knowledge of it.
In the domain of women beyond that first group, friends, close friends, and confidants, it is likely something greater than 90%. However that percentage is influenced by built-in bias, largely due to the circles I run in, including the alternative lifestyle/fetish community, my own wife's long-term infidelity, and the women with whom I've engaged in affairs with. The majority of those women have been married women.
The available statistics are all over the map, varying by country and source. For women, the estimates range from around 20% to as high as almost 70%. The median is something like 48% over European and U.S. married women - based on at least one cheating experience. The breakdown gets even more muddied, factoring in multiple episodes of infidelity, separation, and divorce.
I have lots of personal experience in this realm, as the partner of a cheating spouse, and as a cheating spouse. I tend to believe the median is close to accurate, based on de-biased personal experience.