Quote Originally Posted by Armchair Athlete
Also the game supports a maximum of 300 units.
Some quick numbers, theres 264 units in RTW (no mods) and about 180 in MTW with VI.
Some of those RTW units are repeats though, just using different models (like the peasants).

Now, with (say) 30 factions, that means each faction can have up to 10 units, if every unit in the game was unique to one faction (a bit small). But if every unit that was in MTW:VI was put into RTW, we would still have 120 or so to spare. Whilst you will need more generic units, I think for now is safe to presume you dont have to go overboard with them.
As an example, if you had 30 factions, and 5 culture groups with 6 factions each, if you made a generic peasant unit for one culture group, you would have effectively freed up 5 slots for unique units (if all factions in that group would have used a unique peasant unit instead). I should guess that these 'savings' would add up pretty quickly.