how many teams have used the stadium which hosted the athletics at the 1912 summer olympics – men's 3000 metres team race ?
SELECT DISTINCT COUNT attr_open var_uri attr_close where brack_open dbr_Athletics_at_the_1912_Summer_Olympics_–_Men's_3000_metres_team_race dbp_venue var_x sep_dot var_x dbo_tenant var_uri sep_dot var_x a dbo_Stadium brack_close