While searching for the right transfer switch, you will want to know how your application requires the neutral to be handled. The rules vary by application and in some cases depend upon the authority having jurisdiction (Local electrical inspector).
A solid neutral switch provides basic load switching. The neutral of the generator, utility and building are permanently connected inside the transfer switch. The transfer only switches the hot (power) leads. In this application you need one pole on the switch for each hot lead.
A switched neutral transfer switch only allows the utility or generator neutral to be connected to the building load at a time. Since the neutral is switching between sources another pole on the transfer switch is required.