The following rule of thumb is a good starting point in estimating the number of tables you will need for your event:
Round Tables:
72″ rounds seat 10 to 12
60″ rounds seat 8 to 10
Rectangle Tables
8’x30″ tables seat 8 to 10 when using all four sides, 6 to 8 when you’re not using the ends.
6’x30″ tables seat 6 to 8 when using all four sides, 4-6 when you’re not using the ends.
4’x24″ tables seat 2.