Whatever the event, it is important that there is enough libation to last throughout the party. No host wants to be running to the store halfway through the party to buy more alcohol. It is always a good idea to buy more than you think you need. Most alcohol won't spoil if properly stored, and can always be used when surprise guests stop by. A general rule of thumb is to estimate 3 or 4 drinks per person over a four-hour period.
Don't forget to buy enough ice. Approximately 1 1/2 to 2 pounds per person should be enough when serving mixed or frozen drinks.
The following charts will give you a general idea of how many drinks can be made per bottle.