@Count Blue : I think he meant how many air wings to combine into a unit to make an effective fighting force, etc.
In case that is what you wanted to know, I believe for planes it is about 3 or 4 air wings of the same type per squad.
For ships its a bit more complicated, as you should combine types. The most universally accepted models seem to be:
4-6 BB (Battleships) and 6-8 DD (Destroyers) for a strong SAG (~battle fleet) that will be strong against other battle fleets without CV (Carriers)
2-3 BB and 10-12 DD for a CV-hunting SAG
3-4 CV and 6-8 CL (Light Cruisers) seems to be accepted to be the strongest overall fleet, as CV tear everything to shreds once they reach ~1940 tech
There are more possibilities, e.g. Cruiser-only fleets for speed, convoy raiding and AA (BC, CA and CL) or Anti-Submarine fleets (DD), but many of these are either inessential (i.e. don't make a big difference to whether you win the game) or could more efficiently be done by air units.
For land units it is faaaar more complex, but you can break it down quite easily: The only thing you "need" (unless you are so familiar with the game that you can game the system) is standard Infantry divisions with the appropriate support. I would go out on a limb and say that even a mediocre player can beat the game by using only standard line divisions of the make up INF/INF/INF/ART with one or two INF/INF/INF/AT per Corps. If manpower is an issue for your country, INF/INF/ART/ART works almost as well. You don't really need tanks at all, but if you want to use them, it is a good idea to combine them with MOT and SPART and never use more than one tank brigade per Division. My suggestion would be ARM/MOT/MOT/SPART, but I guess you could go manpower-cheap again and use ARM/MOT/SPART/SPART. Land units always move at the speed of the slowest unit, which is why you shouldn't put standard ART or AT into tank or MOT divisions and at the same time don't waste SPART on INF divisions (unless you plan to upgrade the INF to MOT).