I like scrappy quilts so I use a bit of a LOT of different fabrics. I usually only need to worry about having enough background, border fab and backing, because the rest is all scraps.
Here's a list of approximately how many pieces of each size you can cut from an 18" x 21" fat quarter:
90 2" squares or
50 2 ½” squares or
42 3" squares or
30 3 ½” squares or
20 4" squares or
16 4 ½” squares or
12 5" squares or
12 5 ½” squares or
9 6" squares or