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That's the case but see the thing is if you are wagering over three or say more teams in order to improve your chances of winning you would have to bet $30 for each of them which is little bit more according to me compared to your free bets for each match.
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
That's why if you're going to participate in this promotion you have to think deeply on what country could dominate the world cup to have a chances to get more $5 for each winning. Yes it's a bit hard but if you happen to bet on the right country, that's satisfying. Anyway, probably some of us can't afford to risk $30 (specially the average gamblers) but if you're open for a challenge and a fan of FIFA then it's worth to try, who knows right? Just have less expectation.