There are a lot of great patterns out there that call for fat quarters. You would then use yardage for the borders or sometimes the background.
I just pulled a Yellow Brick Road pattern it calls for 48 fat quarters for a king size (120x120) as well as 3 1/2 yrds for a border. The queen size(93 x 102) calls for 30 fat quarters and 2 3/4 yds for border.
Another favorite pattern I have used a lot for a quick easy quilt is called Bits and Pieces and that pattern calls for 40 for a king (100.5 x118.5) and 33 for the queen (91.5 x 109.5)
It would depend greatly on what pattern you wish to use. the smaller your working units, the more fabric you may need to account for all the seams.
I have, when making a queen Yellow Brick Road, just used 2 FQ of the same fabric and so only had 15 different fabrics in the quilt.
Hope this helps a little