The answer to your query could be in two lines.
12" round feeds 25
14" round feeds 40
Double layer, feeds 60 - 70,
Ok, let me tell you in detail. A 12" cake, in two layers, will be enough to give everyone a slice-but it will be wedding-cake size, not a huge wedge.
When cutting the cake, mark an 8" circle in the center (where the smaller tier would be if it was a tiered cake) and cut around it, so that you have a round piece and an outer ring. Cut the ring into slices and serve, then cut the inner cake into wedges and serve.You shouldn't have any trouble getting 20 slices just about 1.5" wide from around the ring.
A 12x18" half sheet will also work, cut it 6x4 for a 3x3" piece.
Be sure to check out Cake Wrecks to see what NOT to do when decorating!