I do believe the correct answer is thirteen (13)
http://www.retrosheet.org/boxesetc/1...241PHA1928.htm
In the first game of the May 24th double header the following players actually played
Philadelphia
Cobb, Ty
Speaker, Tris
Cochrane, Mickey
Simmon, Al
Collins, Eddie
Grove, Lefty
Foxx, Jimmy
New York
Combs, Earle
Durocher, Leo
Ruth, Babe
Gehrig, Lou
Lazzeri, Tony
Hoyt, Waite
In addition
Both teams had HOF managers and two HOF umpires worked the game.
Pennock was a Yankee but did not play.