According to the AN-2 flight manual:
The minimum number of airplane crew are two persons with current authorisation for performing aviation operations.
Depending on the type of flight the airplane crew shall consists of:
- in flights according to VFR rules - two pilots or pilot and navigator or pilot and flight engineer;
- in flights acc. to IFR rules - two pilots or pilot and navigator;
- in agricultural flights - two pilots or pilot and flight engineer.
Since, in every case, there is the option not to carry two pilots, the AN-2 cannot be classified as a MPA and being below 5700kg MTOW, therefore, can be flown with an SEP class rating, as I occasionally have.