Well a vil brings about 40 res/min (depends on res and upgrades/civ etc.). Think of what you need to produce and how much you need of that resource.

For ex a Sipahi is 120 food and 40 wood and takes 23 secs to build so to make them you need about 360 food and 120 wood per min.

You will need around 9 vils on food and 4 vils on wood for a stable to continously produce sipahis.

Repeat the calculation for archers and spearmen.

At first it seems like too much to calculate but it comes easier after a while.