The only reference in the FCOM is "...when taxi weight greater than 76000 kg, not greater than 20 knots during a turn..."
Some regulations (FAA for example) state, "...at a speed no greater than the average man can walk..."
In theory, less than 72 knots to prevent Autobrake from arming (Although, I can't recall if "Take Off Power being set" is an arming requirment - though I don't think that it is)
Good braking technique (Carbon Brakes) usually requires that you allow the speed to build up to about 30 knots, then apply one brake application to reduce the speed. Then, repeat. Albiet, I don't know many men who can walk at that speed.
Most Company SOP's will state a requirement. Ulitmately, use good judgment to keep things safe considering the local environment, etc.