This is what SFG recommends :
.. look on the back of the seed packet–ignore the row spacing, just look at the plant spacing.
There are four spacing guidelines:
Extra Large – one per square for 12 inch spacing
Large – 4 per square for 6 inch spacing
Medium – 9 per square for 4 inch spacing
Small – 16 per square for 3 inch spacing.
Use the seed packet to find out what spacing your plant needs. A tomato or green pepper plant needs one per square. While radishes and carrots need 16 per square.