I haven't played FoF, but have a familiarity with GGWBTS.
Leadership is central to the latter. Lil Mac (or any Army Commander, and each side has 4) can influence units as follows:
1. If he gets initiative he can move any number of corps stacked with him. Initiative depends on supply status, his offensive rating - Mac 2, REL 4, and whether his theatre commander is activated. Winter hurts initiative and late war increases initiative.
2. In combat (started by moving into an enemy occupied region) a leader only influences a number of brigades (game unit building block) equal to his Command Rating. RELee quickly reaches maximum of 24, Grant hits 20, Lil Mac struggles to get above 16 most of the time. Increases in CR are based on promotions, training successes, battle successes of units under command, and being the winning commander in a battle.
So to answer your question, you can put the entire Federal army in one region (Washington City) with Lil Mac and if he gets initiative he can move them all and help a small percentage of them. However, the game mechanics on leadership depend on more than must that for a good outcome.
If in doubt, try to catch it on sale.
"L'audace, l'audace, toujours l'audace."