The way the pulse function works is the first parameter controls the pulse width and the second controls the number of pulses. If there is more than one pulse, the on time and off time of each pulse will be the first parameter.
If you had a one second pulse, with two pulses, the output would go on for one second, off for one second, then back on for one second.
A twentieth of a second is 50 ms, but to complete a full on/off cycle in 50 ms, the pulse width needs to be half of that, or 25 ms. Each pulse will go on for 25 ms, off for 25 ms, then repeat. Five seconds worth would be 100 pulses
pulse 25 100
BTW, 20 times per second is not humanly possible and can get your account flagged as a bot or as using a device like the Cronus Zen in some games, so be careful where you use it.