count all those whose youthclub performed at the 2012–13 indonesia super league u-21 ?
SELECT DISTINCT COUNT attr_open var_uri attr_close where brack_open var_x dbo_season dbr_2012–13_Indonesia_Super_League_U-21 sep_dot var_uri dbp_youthclubs var_x sep_dot var_uri a dbo_Athlete brack_close