Not sure when this was added or why. I don't see anything in the end game help file about.

How is it determined how many units you can have? Is it based on how many cities you have, or your total population?

Since my ally keeps deciding to go to war with someone new every so many turns, before I've had time to even move my forces to where the first one is at, three enemies so far, I needed a larger military. So I keep having my spies recruit defectors, each successful attempt adding three new units to my fleet.

Is it just military units that count? What about workers, trade convoys, colony units, and satellites?

And why would anyone ever build any military units, when its faster to get three at a time per spy instead of building a single unit in a city?