Typical FMCW radars transmit very short ramps (microseconds) at a very long (relatively) intervals (several ten milliseconds), i.e. a duty cycle of less than 0.1%.

In order to create interference between two radars, the ramps have to overlap pretty exactly, within a few nanoseconds of each other. This is very unlikely to happen.

Modern radars employ technologies to detect and/or avoid such collisions.

Overall it is not really an issue, even with many radars in crowded spaces.