From what I have measured and seen listed. It goes from 2.1 mm (a magura SL drilled I measured) to around 1.7. for a Formula R1 (2 piece)
The others I have seen and/or measured seem to fall somewhere in between.
Keep something in mind, your brakes were designed to use a certain width rotor. If you use another brand, you may have some adjustments to do.
Lead by my Lefty............... right down the trail, no brakes.
My 160mm Alligator windcutters I just installed are thinner than the Avid clean sweeps that came on my bike. I had to go in about 2 clicks on both pad adjusters front and rear and the caliper needed a slight adjustment.