When girls train together, they may train with up to 3 others. So 4 girls total.
4-8-12-16-20.
With this in mind, it's very odd that the 20th girl in both the first and second brothels(which have a max capcity of 20) can only find 1 girl to train with. I'd expect this with the later brothels(which have max capacity 25, and thus a 'loose' girl.
EDIT: done some further checking-it's seems the last girl ALWAYS ends up with 1 partner, regardless of numbers.