Among the players from the ABA league, how many of them have the center position?
"ABA" is the lgID; center position refers to pos =   'C' or pos = 'F-C'; players refers to playerID
SELECT COUNT(DISTINCT T1.playerID) FROM players AS T1 INNER JOIN players_teams AS T2 ON T1.playerID = T2.playerID WHERE T2.lgID = 'ABA' AND (T1.pos = 'C' OR T1.pos = 'F-C')