First let me say thanks for dowloading our rules. I hope you enjoy them!
Now, to answer you questions.
Each unit, vehicle, crew-served weapon, or unassigned Main Character gets a card in the deck. So, in your example of two opposing 10-man units, there would be 2 cards in the deck; one for each unit.
The Main Characters acting as leaders of units do not get separate cards, they operate with the rest of the unit. Only Main Characters who operate as individuals and are unassigned to units, vehicles, or weapon crews get their own cards.
The exception to all this is Main Characters rated as a "Hero". They, or the units/vehicles/crew-served-weapons they directly command, get 2 activation cards in deck.
I hope this helps. Please post if you have any further questions.