Not only did turbo buttons serve a very minor purpose (slowing the machine down so you could play "dumb" games at a sensible speed), on most of them, the numbers they displayed wasn't even measured.
The "turbo" function was provided on the motherboard. There was a 3 pin connector, and the turbo button connected (usually) the centre pin to one of the two outside pins when pressed, and to the outside pin when released.
So how did we get the nice cool numbers to display? Jumpers. A whole load of them in the case. And you had to set for every segment in the seven segment display whether it should be off (no jumper), on when in turbo mode, on when in slow more, or on when in both. This was often done with 4 pins arranged in a small "T" shape. The common pin was the top-middle, and you connected the jumped left, right or down for the three options.
Towards the end of the craze of turbo buttons, some motherboards progressed to where you could configure the display in the BIOS setup screens.
Either way, it was up to the person who built the machine to decide what numbers to display. Most common was the clock speed that the machine ran at when in each mode (from the motherboard documentation), or the Landmark speed rating (measured by a piece of software once the machine had been fully built).