The total amount of force they exert on the box is 500 newton's . despite their efforts, the box will not move. what is the amount of force exerted by the box on each person?

Whatever amount of force each person exerts on the box, THAT's the amount of force the box exerts on him.  There's no reason the forces of each person have to be equal.

Let's say . . .

-- Mr. Smith pushes on the box with 499.99 newton's of force.

-- Mr. Jones is actually a fly; he pushes on the box with 0.01 newton of force.

The total of their forces is 500 Newtons on the box.

The box exerts 499.99 newtons of force back on Mr. Smith, and 0.01 newton of force back on Mr. Jones, the fly.