If it is the only receptacle on the circuit (and 20A breaker and #12
wire) it should be 20A.

In any case, there is no reason not to use a 20A receptacle.

--
bud--

Thanks. It is the only outlet for a wall A/C unit.

And the wire is 12/2 BX. So the metal clad is ground. The box is
steel. I am still going to put a green ground wire tail from the box
to the outlet.