We all no its wrong,but what alot of fanicers do is breed 10 extra YBS to cover for loses.
when i first started our top flyer had 9 wh cocks and 12 ybs, thats a small team ;D.
I think it also depends on what distances you fly, and what methods,and how many clubs you belong to.
Ourselves we will start with 12 pairs and 24ybs