I think the book tells you to look at the seed packet. If it says to plant the seeds 1 inch apart, you plant 12. If 2 inches, plant 6; 3 inches, 4; 4 inches, 3. You do this by dividing the square foot into how ever many sections required and plant the seed in the middle of each section.
But it's been a while since I looked, and I'm at work right now.