I'd think it's a small overlap. There are about 617,000 active pilots and the FAA figures about 335,000 A&Ps but it's a lot harder to track A&Ps since they don't get a medical. Just guess from the people I've flown with I'd say somewhere around 5% but then they may have been more but it didn't come up in the conversation.
I was looking for work a few years back, a company I was talking to wanted a type rated captain. Check. They also wanted the captain to have an F/E rating. Check. What they really wanted was a type rated captain with the F/E and an A&P. Sorry, I'm not your person.