Well it works if it's a melee map (And i think the default number of players is a 4v4 match). Otherwise you need to create a new game variant for a custom map. You'll need to set it up as custom teams, 5v5, 6v6 etc. etc or set up the start timer (Please don't leave it as 30 seconds...)
Because the melee map default is 4v4, if it were anything over or maybe under (?), it would completely screw up your set up. The Editor recognizes this and won't let you publish until it's fixed.