My recollection, clearly flawed once again, was that he had nine children by his first wife, and then sixteen more by his second, making a total of twenty-five.

Somewhere on Peter Shickele's liner notes he identifies which number child was PDQ - does anyone have access to it still?

Edit: The PQD Bach page http://www.presser.com/composers/pdqbach.html sidesteps the issue (no pun intended, of course) definitively :

The conspiracy of silence that has surrounded P.D.Q. Bach (1807-1742)? for two centuries began with his own parents. He was the last and the least of the great Johann Sebastian Bach's twenty-odd children, and he was certainly the oddest...