Need some help with electricmagnetism

[zer0shifter]

Hey so I'm doing a class project involving a sort of generator device. The device will be a RC car wheel with very strong but but small magnets that each have a magnetic field of 3200 guass and then next to it will be a donut like wheel with magnet wire wrapped around it in the oppisite direction so when the wheel spins on the RC car it will generate electricty .The problem i need help with though is i don't know how thick i should make the donut/wheel to wrap the coil around so i generate atleast 12 volts from it. i can give you any other additional information you might need but can someone please help me or give me a formula