It depends upon a number of variables such as size of brooder, type of material brooder is made from (wood, metal, plastic, cardboard, etc), Where the brooder is located (house, insulated garage, uninsulated garage, barn, etc.)
A metal brooder in my insulated garage last year required (2) 250 watt heat lamps for the first three weeks. I replaced it with a large wood brooder this year and I am going to get by with (1) 250 watt heat lamp. Initially it will be heating a 3' x 3' area. After about two weeks I will remove the dividing wall and it will become 3' x 8'. I tested both configurations last weekend. The 3' x 3' area heated nicely to 96 degrees. The 3' x 8' area heated to 85 degrees. (Temperatures measured near the floor under the heat lamps).
I am using a red heat lamp.