Mach Number:-

It is defined as the ratio of actual velocity v to the sonic velocity c

$\therefore M=\frac{v}{c}$

when

$M\gt 1$.......Flow is supersonic

$M\lt 1$.......Flow is subsonic

$M=1$.......Flow is sonic

Stagnation pressure:-

The stagnation pressure $P_0$ is related to mach number and static pressure in the case of an ideal gas.

$\frac{P_0}{P}=(\frac{T_0}{T})^{\frac{\gamma}{\gamma -1}}=(1+\frac{\gamma-1}{2}M^2)^{\frac{\gamma}{\gamma-1}}$