If Newton tried to derive his law purely from empirical measurements, then yes, he would never be exactly sure (ignoring general relativity for a moment) that the exponent is exactly 2. For all he would know, it could actually be 2.00000145...

But that would be like trying to derive the value of pi or the exponents in the Pythagorean theorem by measuring physical circles and triangles. If the law of gravity is derived from more general axioms, then its form can be computed exactly provided that these axioms are correct.