Is it possible to determine the mass flow rate through the engine of MiG-21 during its various cruise Mach no. operations?
The mass flow is approximately $20 \ldots 100~kg/s$.
You can try to calculate it yourself using thermodynamic equations (see for example NASA Resources, MIT Resources or dedicated books).

Although the (idealized) calculations are not very difficult, please note that you will have to make a lot of assumptions concerning the efficiency of the turbojet components (particularly intake, compressor, combustion chamber, turbine and nozzle), since this data is not published by engine/airplane manufacturers. Some variables that are usually available are the thrust, the total compressor pressure ratio and maybe the (approximate) geometry of the intake and the nozzle.
Also, you are asking for supersonic calculations, which are more complex than the subsonic ones -- including the assumptions.