We have an ice machine that produces ~400 lbs of ice in 24 hours.   The rule of thumb is 2.5 to 3 pounds of ice per guest.

In general, you should purchase ice if your guest count is greater than 140. 

Bottom line: Tell your catering and bartending staff the amount of ice available, and let them make a determination.  Ice is inexpensive.  If you have a bag or two left over, you can leave in freezer and pay it forward!  A much better approach than running out!