A 48 " diameter circular table can comfortably fit 4 or 5
A 54 " table 5 or 6
A 60" table 6 or 7 tightly
A 72" table 8 or 9 tightly

With smaller tables you need a little extra space per person but the formula is that each person always needs at the very minimum 24" of space to sit at, and 27" is comfortable. The circumference of a circle is the diameter multiplied by 3.14 (PIE) Now divide by 24 or 27 and see how many fit. Don't try to squeeze a person in if it doesn't make the 24" minimum.