OK, these are the issues.
I've got a lot of chariots, but they're not all in the right place.
Persian chariots? All over the place from about 5 incompatible sources. Assyrian chariots 3 incompatible sources. The Persians - I can 'afford' to lose some. The Assyrians - really, they group into 3 separate forces anyway, one of which is very short of chariots. The Minifigs 'S' Range figures only came with 3 chariots:
Well, it's a couple of things to ponder about.