Here is an idea, which also creates balance.
States get forces based on the following:
For each World 10% of a regiment
For each Mech or Aerospace line 1 Regiment
For each other type of factory line 25% of a regiment.
So a state with 100 worlds, 10 lines of Aero and Mech and 20 of other industry would have 25 regiments.
Or use whatever percentages work for you.