Total of 20, though only 13 of them are current.

Current stadiums:
NL East: Mets, Nationals, Phillies
NL Central: Cubs, Brewers
NL West: Giants
AL East: Red Sox, Orioles, Blue Jays, Yankees
AL Central: Indians
AL West: Angels, A's

Retired stadiums:
NL East: Mets, Phillies, Nationals, Marlins, Expos
NL Central: Pirates
AL East: Yankees