Among the players from the NBL league, how many of them were born in Spencer?
"NBL" is the lgID; 'Spencer' is the birthCity
SELECT COUNT(DISTINCT T1.playerID) FROM players AS T1 INNER JOIN players_teams AS T2 ON T1.playerID = T2.playerID WHERE T1.birthCity = 'Spencer' AND T2.lgID = 'NBL'