Yaw rate of a multi-rotor
A conventional quadcopter yaws by speeding up two rotors and slowing down the other two.  Once those two have slowed down to zero RPM, it can't yaw any faster.
A formula for this would involve the rotational moments of inertia of each motor+rotor and of the quadcopter's chassis, and the distance from the quadcopter's center of mass to each rotor's axis.  (If the rotors aren't coplanar, it gets messy.)
If those two rotors can be reversed, then they are (inefficiently) thrusting up instead of down.  The limit then may be when they overpower the thrust of the first two, forcing the quadcopter to descend.  Or, if the rotors are spectacularly inefficient when reversed, the limit is when the rotors reach maximum RPM.