500 raindrops per second means that you have 500 times 0.00005 kg striking the windshield per second.
So in one second 0.0250 kg of mass at 16 m/s strikes the windshield. This creates a change in momentum of 0.4 kg m /s in that one second. This is, in turn, equal to the impulse exerted on the windshield in one second: 0.4 Ns. Thus the force on the windshield is 0.4 N s / 1 s = 0.4 N.