Union wise we usually calculate 5 crews per shorthaul aircraft and around 10 to 12 per longhaul. MFF between shorthaul and longhaul is a bit more difficult as it depends on the ratio between long- and shorthaul flying, but somewhere between 7 and 8,5 crews per aircraft should be ok.
In reality you need a few more captains than FOs as most trainers and nearly all management pilots are CPTs and can only fly part time. However longhaul which augments crews with senior FOs needs more FOs than captains. All in all not allways an easy topic.