Database: professional_basketball
Question: Among the players from the ABA league, how many of them have the center position?
SQL: 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')