Incredibly common. I haven’t seen any recent estimates but I recall at one time that ~15% of children did not have the biological fathers than they thought they did. It’s not to point the finger at women - rates of infidelity may be even higher among men, it’s just harder to track independently.
This wasn’t a published study. This was beers with the folks in the genetics lab at the hospital I worked at. No patient names or other identifying information was involved. They did tissue matching etc. and ran into the issue all the time. On a personal note two close friends have found out they had different biological dads than they thought they did. So maybe my perspective is skewed somewhat.