Certainly a complex subject, there is a good book titled "War in the Middle Ages" by P. Contamine. It goes extensively about the warfare organization of the time. As an example the different princes, counts etc in Central Europe would provide a number of Knights for a limited period every year to attend military business. So based on wealth, size etc a small one may provide for instance 2 fully equipped Knights, squirrels and 3 sergeants for a period of 7 weeks who should proceed at a given rendezvous. This is for the sake of the argument, can't remember exact figures on top of my head as I read it years ago. This was then funded by that Lord, on top of those 7 weeks the King asking for the services should take the cost of ongoing operations, etc. It's a fascinating insight into the management of warfare, I strongly recommend it.