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.
The ice is only for the bar, and caterers on our list know that they need to bring for their own.
In general, you should purchase some additional ice if your guest count is greater than 150.
(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!)