Mach is the coefficient of the speed of sound, so Mach 1 means
you are travelling at 1 x the speed of sound.
View page
The mach number is the speed, compared to the speed of sound in
air. Thus, mach 1 is the speed of sound, mach 2 is twice (2 times)
the speed of sound, mach 3 is 3 times the speed of sound, etc.
View page
Mach 1 is about 762 mph, so mach 20 is about 15,240mph.