The number of candidates historically taken to waterside is generally 3 x the number of places for each FTO.
First year: 90 places in total, 30 per FTO. Each FTO sends 90.
Last year: 72 places in total, 24 per FTO. Each FTO sends 72.
This year: 60 places in total, 20 per FTO. Each FTO sends 60 ?
The cost of selection only covers the FTO selection costs (and maybe a bit of profit too?). So the costs of the day at Waterside are covered by BA. The purpose of the FTO is to filter out the candidates, so I can see why BA want to limit it to the best x candidates.
Maybe FTE get some brownie points from BA for doing a more rigorous selection procedure and therefore send less than the limit? There are dangers in that too though..