Watch the units ie not mixing up Nautical miles and metres.
Max Range = C spd of light divided by (2xPRF)
Turning the formula around :-
PRF= C divided by (2 x max range)
Speed of Light = 300 Million metres / second
Distance = 50NM= 50 x 1852 metre = 92600 metres
PRF = 300,000,000, / 185200 =1619 pulses per second