Torque is a measurement of rotational force. Brooms would technically have thrust. If I remember my high school physics, a Broom would need at least 980 Newtons of thrust to liftoff your average sized witch.
Assuming that's it's just the normal force of Earth's gravity at work on the witch, it's just a case of working Newton's second backwards with the gravitational "constant" acceleration ≈ 9.8 m/s²:
F = ma
980 N = m × 9.8 m/s²
980 / 9.8 = (m × 9.8) / 9.8, units omitted for text clarity
100 kg = m
In other words, op is assuming that an average size witch masses approximately 100kg (or about 220 lbs in fingers, knees, and toes units).
So no torque, then? Unless the witches are rotating so fast we can't perceive it with the human eye, like a car's wheels travelling alongside you at a particular speed.