What's the relation between TAS and Mach in tropopause?
The speed of sound depends only on air temperature, not density. So, for a given temperature, a given Mach number will always equate to the same TAS.  

In other words, for a given Mach number, TAS remains constant throughout the constant-temperature region above the Tropopause (ie. 37000ft to 65000ft in the International Standard Atmosphere, ISA).