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.
I am sure many people could afford $30, that is not a lot of money even in the poorest nations to be fair, not small change neither, depending on where you live that could be as high as a weekly grocery store, or an electricity bill paid, but it is still not "a lot" in the sense that it is not rent money in almost any nation, maybe 1-3% of the world would have that as rent.
However, it is still not a smart decision to put that into world cup if you don't have it. I will certainly try it, England and France and Brazil, these three look like they could actually do something and it really feels like we could end up with a good return if we wager on these three nations.