Welcome. Civ II, usually, each city could only support 3 units.
Though I have heard that on Chieftainyou can go in the red and it wouldn't matter, so, technically, all units could be free then.
Haven't tried that yet though. Imagine having 500 tanks, 200 destroyers and battleships, 300 marines, -1000 gold each turn and not losing anything. Sure, you couldn't rush anything but hey, when you're overrunning your opponents with unreal amounts of units, who needs rushing?