How many electrons can occupy a single molecular orbital?

Question

and Alumimum is a group 3A metal, Which ion does A1 typically form? I'm thinking Al3+

Your first question depends on the molecule involved and how it is bonded. Hybrid molecular orbits can be formed.

Now in an atom, there are electron orbitals of mostly fixed shape and predictable energy levels and vacancies.

In a particular energy level there can be up to 2n^2 electrons. In a particular orbital, and there are in any energy level, up to 2n-1 orbitals, or a total of 2(2n-1) total electrons in that energy level.

I recommend you see your text on this, I suspect there is a chart showing the filling of the electrons into energy levels and orbitals.

http://en.wikipedia.org/wiki/Atomic_orbital

Yes, aluminum forms the +3 ion.