Just a word for all those assembling cavalry units for Fire and Fury, there isn't much used. You are better assembling even more infantry. The cavalry were really under used in the civil war. From the base sizes in the rules the average brigade is 7 bases. They tend to vary from 2-12 base for an infantry brigade (read regiment for brigade).

I reckon accounting for unusable figures etc that from a box of fifty figures you can get a brigade and a half to two brigades depending on how many figures you put on a base. For the Confederates there are roughly five brigades in a division. Union divisions tended to be between 2-3 brigades in strength. Both sides made their corps from 2-3 divisions. I hope this gives some idea of how many boxes you'll need.