My point is not that you need the number to be exactly accurate, my claim is that for the number 2 you don't even condider the student body as a quantity, but f

My point is not that you need the number to be exactly accurate, my claim is that for the number 2 you don't even condider the student body as a quantity, but for 20000 it very much becomes a quantity. Also, if 20000 students need training, as a teacher you can still provide training, you just need to do so in other ways. Either, you can recruit more teachers to help you (and then it matters if the number is 10000 students or 20000 students), or you can switch to another medium (a book, a video, on-line training). Treating a student body as a number will make it more managable for you to provide/organize training to them than just hireing more teachers at random until each student gets enough attention. Likewise, 2 trees may keep you warm for a week, 20000 may keep you and half your (small) village warm for a year, with enough trees left over for the forest to replenish.