When you refine a single cell you get 8 cells because you split it along the x, y, and z planes. This gives you 8 smaller cells. Now for the 2nd refinement each cell becomes 8 again. So 8x8=64.
let's also remember that there is a mesh conservation that gets applied afterwards, so this is the theoretical number, it may not be that. see the tech reference for more info on this.
64