at the sale prices, I would get at least a yard of each fabric choice...or more if you can afford it!!!! BUT...be sure to WASH AND DRY AND IRON each piece of yardage BEFORE you cut it because flannel SHRINKS...don't ask me how I know this to be true...
as to how much to buy....deceide on your colors and then get maybe 8-10 yards for a nice size quilt. remember, there are 2 layers of flannel plus the batting in a rag quilt and you will have a lot of shrinkage with the washing / drying....always better to have more ( leftovers!!) than not enough at those prices!!!! they won't come back until the next Black Friday