Boeing 747s can fly at Mach 0.92, but they typically cruise at Mach 0.88. This speed changes based on the plane's altitude, which affects the density of the atmosphere.
Boeing 747s are typically flown at speeds lower than their maximum. This is largely due to economic reasons; lower flight speeds are more efficient, which lowers the cost of flying them. Safety factors matter as well. If an airplane flies too close to the speed of sound, it may become damaged, and its control surfaces may stop working. By flying at a lower speed, pilots are better able to maintain control if the plane enters a dive.