Welcome, John!
There isn’t really a “formula” for figuring this out. The setup incorporates other things besides the simple number of troops available to each power. It also takes into account things like the quality of leadership and war materials available to each power. As a result, an “infantry unit” may be a different number of men in different instances.
OK and how about all those units in Southern Germany, Czechoslovakia etc.? Weren’t they nearly all involved in the attack on France and the Low countries at the beginning of the game? (along with a screening force in Poland, some forces in Norway etc.)