The gpu receives power from the connectors on the back and from the traces on the motherboard.  The motherboard that I use has a 6 pin connector on the motherboard to power the pci-e slots.  I do not use powered risers on a 6 card rig.  If you do not have a power connector on the motherboard specifically for the pci-e slots you need to use powered risers, because the motherboard traces and the wire feeding them aren't thick/wide enough to carry the power for all of those cards