To determine how many tables and linens you'll need, first you need to know what size tables you are going to use. If the venue requires you to rent them or to supplement what they do have, there's any easy way to figure out what you need.
Tables used for events are typically called Banquet or Round.
Most often referred to by their shape and length with the most common sizes being 6 foot and 8 foot. An 8 foot (') table is commonly called an 8' Banquet table. When you hear banquet table, HEAR rectangular. These are typically used for buffets, beverage stations and sometimes, for seating.
Round tables are called by their diameter in inches (") so a 60" round table is called 60" round. The most common sizes for round tables are 48", 60" and 72". Larger round tables are often used for guest seating while the smaller ones are used for Wedding Cake tables, sweetheart tables and childrens' tables.
Notes from a Pro:
When you speak about your head count, you should mention how many people are in the wedding party - including the bride and groom - along with the guest count which combined is your total head count. (They probably all need a seat and they all will want some food).
Wedding Cake Table:
Be sure to plan your cake table size according to the size cake you're having. Use the diameter of the largest tier (bottom layer) to guide you.
Now, on to the Linens...