How to determine poles?
Synchronous speed RPM = 120 X frequency / poles. With 4 poles and 1500 RPM, you get 50 Hz.
For a 3-phase motor, you would distribute three sets of phase windings in the stator slots to make a pole. There are various schemes for distributing and overlapping the windings to do that.
For a squirrel-cage rotor, the slots are filled with cast aluminum or copper bars and the ends are connected to copper or aluminum rings at each end. For a wound rotor, the windings are distributed similarly to the stator.