Most load centers have this info. on a sticker inside the enclosure door.Typically, 2 ground wires of the same size, but only one neutral. How many times have we seen(and done) the neutral and its ground in the same bus hole. Probably a violation. Breakers, also. The set screw type typically accomdate only one. Some of the others (Square D) have a pressure plate under the screw head that will take one wire on each side of the screw. As far as wiring devices, one screw, one wire.