While electrical machine designers typically go to great lengths to choose the optimal number of stator slots based on the electromagnetic performance, often the thermal aspects of this selection are overlooked. This article investigates the multi-domain impacts related to the slot number selection, combining both the electromagnetic and thermal aspects. Taking an existing 42-kW 4-pole, 12-slot electrical machine with Halbach-array surface-PM rotor, used within a more-electric marine engine as a case study, performance aspects of the machine with different slot numbers are investigated. The impact of slot number is compared considering the losses, output torque and its quality, as well as the peak temperature. Experimental tests are performed, verifying significant temperature reductions and performance entitlements when the optimal stator slot number from thermal point of view is selected. Finally general guidelines on slot number selection for temperature reduction are provided.