More than might be suspected if you followed MN, I think. Mine, but it is a work in progress; my best friend's - she threw him out, he went to live with OW, reality dawned on him, and he begged to come back, but they are still in counselling; my parents'; the two female work colleagues in my office and many more. But equally I know a good many that didn't and usually it was because the DH or DW - but generally it was the DH - left for the OW. Of those I know about, the vast majority didn't end up permanently with the OW/OM but either the damage was too profound to be repaired, they didn't want to go back to their marriage, or their DW/DH didn't want them back.
My sister-in-law's mother has just died of cancer and on her death bed said she profoundly regretted leaving my SIL's father for her OM, who she had gone on to marry, had regretted it for years but hadn't had the courage to admit that she had been wrong, that it was not the profound 'love' she had thought it was during their initial affair and that she would do anything to have her time over again and have returned to her first husband when he begged her to do so.
I have a friend who has had a spasmodic affair with a man for 20 years. Neither have any intention of leaving their partners and unless something changes dramatically, I imagine that it will be one of those 'undiscovered and undisclosed' relationships. I think it's thoroughly awful and dishonest but they are both quite comfortable with it.
My mother said the other day, when I mentioned about friends who seem to have got over a rough patch in their marriage caused in the large part by their profoundly disabled child and their mutual exhaustion in heroically , trying to care for her,her siblings, work etc, that 'like many couples do, they seem to have reached an accommodation with each other and their situation'. I think that is what happens when affairs strike at the heart of a marriage/relationship - you either go your separate ways, or you find that 'accommodation'. The route to it varies for everyone - I have been a nightmare to live with for my H, whilst my best friend has been a paragon of resolution, calm and common sense, but we have each in our own way, got there.
But I have no doubt, that apart from losing one's spouse through death, or - god forbid - losing one of our DC, this is the worst pain I have ever , or will ever, endure, so I have no illusions about how hard the route travelled by so many is.