Actually I wonder what cultures we'll have? I would think
Barbarian = Norse (nordmannaland, danmark, sveariket,rus)
Roman = Catholic (can we only assign 3 + 1 factions to this?). Because of restrictions, catholics would be HRE, French, Loth, Papals, Normandie, Asturia, Welsh and Saxons? Or should welsh be gaelic and saxons norse maybe?
Eastern = Steppe (Bulgars, magyar, Khazar(?))
Greek = Greek (Byzantine)
Carthagian = Muslim (abbasid, al andalus)
And if this is an own culture:
Ptolemaic = Gaelic - if not, gaelic has to be norse or catholic. (alba, eire)

When it comes to BI, I doubt there will be more cultures than this but I've got no clue.

I think around 15 units per faction would be decent (15*20 = 300), allthough this is a hell of a lot of work for two skinners/modellers. So approximatly this could be the case:

Norse: 15-20 shared units (ok I agree this is a lot, but we do focus on vikings), maybe 2-3 uniques
Catholic: 10-15 shared?
Steppe: Should have around 15 shared I think
Greek: lots of zor, counting around 20
Muslim: should be 10-15 shared
Gaelic: I think 10-15 shared

Oh by the way, when I say shared I mean on model and stat basis, not skins. I may have misunderstood.