Yeah it’s really not clear. Last game I built me up to have 5 army commanders and 1 fleet commander and they were parked on cities with no units in them at the end of explo. and I had 7 ranged, 7 infantry, 2 cavs and 6 naval untis spread around the place.
When modern started, i wound up with 4 ranged, 4 infantry and 1 cav spread around the city centers, 3 army commanders with no units, 1 with one cav one inf and one ranged, and a final one with 2 inf and 2 ranged. plus my fleet commander had 4 ship of the line.
so globally i went from 6 to 4 naval, 16 land units to 16 land units. my guess is the game does NOT create new untis for you, but if you have enough commanders to house your units, you will keep them all. Hence the three empty army commanders. and I guess that if I’d had a second fleet commander that one would have had two naval units in it