Here is a chart of plants and how many to plant per square. See below for explanations on how to calculate and sample pictures.
To calculate how many plants per square, look on the back of the seed packet–ignore the row spacing, just look at the plant spacing:
3″ plant spacing = 16 per square
4″ plant spacing = 9 per square
6″ plant spacing = 4 per square
12″ plant spacing = 1 per square
3″ plant spacing with trellis = 8 per square
Some plants require more than one square.