Single up used to be advocated to find the pigeons that could navigate and to train them to be happy flying solo. Double up or triple up was advocated to give the pigeons a flying mate to race against thus speeding them up because single tossing definitely produced lower velocities. With this in mind, the larger the flock the faster they would fly once they build a 'head of steam', especially so with sprint style racing. With regards to training, do not put all your eggs in one basket. Train your pigeons in small batches to avoid complete loss from BOP attack. These are all comments that I have heard mooted by others, you decide, bearing in mind they only take into consideration numbers and no other factor, excepting the BOP attack.