Interestingly enough, no exact number of participants is given (unless I missed that, too), only that only 75% got past the pre test and that all of those teams made it into the Forest of Death. Could be 12 teams, could be 20...
What we do know is that there are twice as many delivery scrolls than final scrolls in play now.
Let's play with the numbers a little and start with 20 teams. I'd expect at minimum two or three teams for each major village plus one team for each minor village. Minus 25% failing the illusion test.
Three teams have three scrolls, some teams will only have one and most will have two. (I'm being generous here). So two scrolls per average, giving us ~40 delivery scrolls.
That means there are about 20 final scrolls within the forest, giving us a maximum of 6 teams that could advance.
It's probably going to be fewer though. Some scrolls might get destroyed (or opened), thus rendering them useless. We don't know if there's an even distribution of final scrolls although making one kind rare would be cruel.