How common is it for flight attendants to have pilot certificates?
In my experience, it is uncommon but certainly not unheard of. Maybe between 1% and 5%, as a ballpark.  A flight attendant married to a pilot who owns his own aircraft has access to the plane and instruction, so that's one scenario. In another case, an F/A I flew with a few times was building time as a CFI and doing right-seat work at SimuFlight, before she left our airline to start with a regional. (Wouldn't be at all surprised if she's flying at a major right now.)

Among flight attendants, there are a lot of really interesting stories of how they got to be where they are. Sometimes the generalizations fit, but when they don't, you find an amazing breadth of experiences in that group.