This is how I think the Kage Bunshin (shadow replication) work: You use chakra to create the clones, to manifest muscles and bone. You then divide chakra between the clones so they all have a small chakra supply. They do, however, get your full physical power. This is how I think (just me) the technique work. He does not get more physical power of the Kyubi, only tons of chakra (chakra can enhance your physical strenght though). More bunshins wouldn't be stronger than normal Naruto bunshins. Still, this is only my theory.