I don’t mix them, but I know people who do, so it’s just a question of taste, I think. All my armies are offensive units.
It takes three armies to build the Pentagon.
The number of armies you can have is limited by the number of cities you have - you can create 1 army for every 4 cities. Given that the max city limit is 512(511 for you , 1 for AI) cities, you could in theory make 127 armies if you wanted too.
Ah, Civ stories. I remember playing the original and deciding to nuke everyone, everywhere. Naturally, the pollution turned just about every square into swamp, which meant my cities soon starved down to near-nothingness. Only one city had enough food to support a settler unit (they had two fish ocean squares), so my single covered wagon set forth to clean up all the pollution and replenish the Earth. It took 800 years.