No dude, there can be as many units as you want in the city. You just load the active one by pressing L. It's just like boarding a ship, but the unit can't get off of it.
OK, example.
You move Napoleon to Paris and press B. Army made. You move an infantry to the city and press L. That infantry is now in the army. You right click on the city and wake up two infantry units that are garrisoned there. The first one becomes active, you press L, it's in the army, same with the second. You now have a full army, ready to guard your artillery or make an assault on a city. It's better for guarding artillery. Swordsman armies are pretty good in the ancient period, tho.
