You need 9 charms to make a block, which when finished, will be 13" finished. I like lap quilts to be around 60" square, so without borders, I'd do a 5 x 5 block layout, which would make the quilt about 65" square. You would need 25 blocks made from 225 charms. Since most charm packs come about 40 to a pack, each person would need 6 charm packs.
If you wanted to do a 4 x 4 layout and use 6 1/2" borders (cut @ 7" ), you would need 16 blocks made from 144 charms, which would only be 4 charm packs plus 1 7/8 yardage (cut border lengthwise to avoid piecing border. This would give you enough extra fabric to cut 26 charms, if you wanted the border repeated that much in the D9P blocks and would allow you to get by with only buying 3 charm packs plus the yardage).
If piecing the borders, you could get with 1 1/4 yards of border fabric (cut 6 strips @ 7" ), but this way you would still need that 4th charm pack.
Hope these help!