There are a lot of things that can be said about what mach is the speed of light, but it’s important to remember that the answer is simply “it depends.” In short, the mach number is a ratio between an object’s velocity and the speed of sound in the medium it’s traveling through. This is not a constant like the speed of light (which is, as mentioned, 299,792,458 m/s in a vacuum). Light travels much slower in glass than in air, for example.
For this reason, the mach number is not a unit of measurement, but rather an expression of speed relative to that medium’s speed of sound. This is why it’s not a good idea to try to compare a vehicle’s mach speed with the speed of light, since it would require dividing by zero, which is not possible.
The mach number is also useful for aircraft, which travel at speeds close to but faster than the speed of sound. These are often called supersonic speeds. For example, commercial airplanes typically cruise at speeds between 244 and 257 meters per second (878 and 926 kilometers per hour), which is roughly Mach 1.
However, the mach number will vary depending on the type of air you’re flying through and its temperature. The speed of sound varies considerably from planet to planet, too. For this reason, it’s important to use an atmospheric calculator when comparing a plane’s mach speed with the speed of sound on different planets and altitudes.