Most rearends take around 3 quarts to fill completely to the "level plug" located on teh side of the housing, usually it's a inverted 3/8 square drive.
In race trim though, most guys will cut that amount by about a quart, hence the 2 quarts recommend.
I run 2 qts in my dana 60 and its alot bigger than a 9 housing :wink: