What is the mass of a person that weighs 500 Newtons?

That would depend on where the person is at. Different celestial bodies have different accelerations due to gravity, which affects the weight. If the assumption is that the person is on Earth, where the acceleration due to gravity is 9.81 m s-2:

Weight = mass x acceleration due to gravity

500 Newtons = mass x 9.81 m s-2

mass = 500 / 9.81 = 51 kilograms

Note that mass is constant, no matter where the object in question is. Weight, however, depends on the location of the object.