How many structures are possible for a octahedral molecule with a formula of AX5Y?

There's only one. It doesn't matter where you put the Y, you can transform it to any of the other positions by just rotating the molecule, so they're all the same structure. There are two possible structures for AX4Y2, because the Ys can either be next to each other or on opposite sides of the central atom, but for AX5Y there's only one.