The small one I used is the same size as my Premier heat plate. I fit 21 under there with no problems. The small one is 12x15 and the large ones I just bought are 12x24. If you go with a smaller pad, I would say at least 4 for your broilers, maybe 5. For your layers, at least 2, maybe 3.
Honestly, that seems like a lot of pads to have to buy. I don't know if I wouldn't use a lamp for the broilers and a pad for the layers.
Edited by henless - 3/5/16 at 6:16pm