Pretty sure that it is limited to only one, otherwise people would be abusing this by betting all those potentially top 4 teams but still though getting a team to win 6 games to get $30 back in free bet is hard so just treat it as an extra promotions though. You might potentially win your outright bet alongside with all those free bets from the promotions
Don't know if it's limited to one only but yes we need to treat it as promotional bet event where you have more chance of winning back the amount while in normal games you don't have idea at all you can win or not but with these sports bet you can place them according to the best team in your mind if you have the knowledge about teams and their playing.