I've never actually gotten a Sunbeam, cheap no-name ones seem to work just fine for me. As I only brood very small batches I find the 12x15 size to be perfect for 3 to 4 chicks, with plenty of wiggle room.
Use what works for you. I have used a heat lamp before and it burned so hot it melted off the red coating - not good! But I do still keep it in reserve in case of emergency. Since OP was specifically asking about pads (I have no experience with plates, for example) I answered about pads.
I'd still recommend 2 - and that would apply to plates as well. Assuming that is their sole source of heat they all need to be able to comfortably fit under it and make contact with it to stay warm.
Generally heating lamps are better for larger groups because they can warm a wider area than a...
It really doesn't need to be that hot, and if you had a heat bulb too they just chose the heat source they preferred. Doesn't mean the pad wouldn't have worked as the sole source of heat.