Hm it is weird.
I edited the Minimap in order to test it.
Now the strange outcome :
A commander can have 999 Units per Squad.
For simplicity i made Banelords with 4 Squads a 500 Vine Ogres.
If i do 16 such banelords everything works fine.
With 17 too. 17 such banelords are 17x2001 units = 34017 units.
If i do though 20 or more Banelords only 17 banelords are shown and the last one is empty.
So the unitlimit is somewhere between 34000 - 40000 units for one nation.
In my Faerun game there were never that many units though, rather around 10-20k in total.
So my bug is probably something else
.
If you have that many units there is another bug too :
You can only see around 5000 units in the army setup screen.
If you deploy them in the garrison though you see another 5000 units etc. so you see always only around 2-3 Banelords.
That could become a problem in a normal mp game too, at least for a successful AE ermor on faerun.