Actually, going over the Recruitment Booklet, it looks like there were 19 houses involved in Formal. So that would be the 18 GLOs listed earlier, plus 4-H House. Which makes sense, since 4H House is listed on the map included in the Booklet, along with the 18 houses. Which is weird, because it looks like they have about 50 girls in their house, which is even smaller than what quota should be this year, plus they are a "cooperative," which would maybe not be as attractive to the majority of NPMs.
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