Why are the artillery in this game 12 guns (2 batteries)and in Napoleonic games just one battery?

Don't know, but if you ever play Bloody Big Battles the gun base can vary from 12 to 30 guns depending on the size of the battle.

I have had another think about this. If a Polemos base represents an entire regiment then only four bases would be needed to represent a division. It then makes sense that the two French divisional batteries be represented by one base and the four Prussian divisional batteries (two 4pdr and two 6pdr - although they were in reality kilos) should be represented by two bases. Not sure how the one battery of mitrailleuses fits into this scheme.