Depends on the fabrication technique. I think using solid state fabrication process we can go upto 50% of reinforcement and even more. 

Thanks Dora Siva, I intend to use stir casting method

It is difficult to produce nano composite in stir casting for volume fraction in excess of 2%. The nano level particles increase the total surface area to be wetted by the liquid metal. The fluidity of the melt also reduces drastically by nano particles. One can reasonably cast upto 20 vol.% micro level particles using stir casting.