If the number of poles are increased for the "BLDC Motor" during its design, the torque density of the motor is going to be increased and also the iron losses are going to be increased eventually. Transistor losses also increases with the increase in poles of a BLDC Motor. Also higher PWM frequencies are required with more poles.

If the no. of poles increases then the electrical frequency of the motor increases with reference to the mechanical speed of the motor and it also reflects in the increment of the iron and copper losses.