Heptaphase electrical current
Generally the fewer conductors you have to route the better. Biphase requires 3 conductors, triphase also requires 3 conductors (when balanced), so triphase is used for preference.

Triphase is already enough achieve constant torque in motors and generators, so there seems to be little incentive to increase the number of phases.

DC bus rectifiers do use hexaphase, and even duodecaphase (12), as these are easy to make from the existing triphase supply, giving a steady DC supply from the rectifier.