How should you decide on the number of sweeps to be used in FMCW radar?
The basic performance requirement of FMCW radar is to measure the radial distance and velocity.

The number of sweeps or linear sweeps relates with the velocity measurement, thus if for a constant integration (processing) period you have more number of chirps (which can be achieved by decreasing the sweep duration) the max measurable velocity is increased.

For the case when the number of sweeps are increased by increasing the processing (integration) time the velocity resolution is enhanced. 

In order to read more detail about the basic of the FMCW parameters, I would recommend to go through this paper FMCW Signal Processing