how many companies have launched their rockets from the cape canaveral air force station launch complex 15 ?
SELECT DISTINCT COUNT attr_open var_uri attr_close where brack_open var_x dbo_launchSite dbr_Cape_Canaveral_Air_Force_Station_Launch_Complex_15 sep_dot var_x dbo_manufacturer var_uri sep_dot var_uri a dbo_Company brack_close