It should be 20 steps per second on GM speed.
Tibia has a server tick (sometimes called server beat) of 50 milliseconds. Your character's speed and the friction of the floor you are walking on determine how many ticks it takes to walk one step. GM speed is the fastest possible, which means you take one step every server tick (50 milliseconds). This results in 20 steps per second.
For reference, the speed breakpoint before GM speed is one step every two server ticks (100 milliseconds, or 10 steps per second).
Be aware that because of step lag, you are unlikely to actually walk 20 steps per second unless you have very low ping (ideally less than 50 ms).