Question

Why 2 Al(s)+Cl2(g)->AlCl3 why its not 2 Al(s)+Cl2(g)->AlCl2

Answer 1

First lets notice it's a simple composition.
So we have \[2Al + 3Cl_2 -> 2AlCl_3\] this should be your reaction, because Al has 3+ ions, and chlorine has 1- .

Does that make sense?

Answer 2

So when you have a compound you must also balance that out, Al 3+, Cl 1-, hence AlCl3