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I found this information online.
A 250-watt heat lamp suspended 18-24 inches over the brooding area that is completely surrounded by a draft guard 12-18 inches high will brood 75 chicks at 50F minimum room temperature. (If the minimum temperature is higher, you can add one chick for each degree. If the minimum temperature is lower, subtract one chick per degree.) This method works very well, but is absolutely dependent on the presence of an effective draft guard, which you can make out of cardboard, plywood, roofing paper, or whatever.
The heat lamp must be high enough that all the chicks can sprawl out in its warmth. If it's too low, they'll push and shove to get into the beam. If it's too high, they'll let you know because of the ear-splitting peeps the emit when they're cold.
Make sure the bulb can't fall to the ground. It can set litter on fire if it comes within a few inches of it. Hang it with a chain, and arrange it so the cord acts as a safety line in case it falls off the chain somehow.