Things like this are defined essentially by the manufacturer, not the NEC. The panel's label would typically have these specs, and perhaps the little sheet that comes with the ground bar as well.
IIRC on most Square D you're allowed two per hole.
Square D breakers (QO and Homeline) also typically allow two wires per clamp for the "screw tab" type terminations... you can clearly see indentations where a wire goes on each side.
