Affine Cartesian codes were first discussed by Geil and Thomsen in 2013 in a broader framework and were formally introduced by López, Rentería-Márquez and Villarreal in 2014. These are linear error-correcting codes obtained by evaluating polynomials at points of a Cartesian product of subsets of the given finite field. They can be viewed as a vast generalization of Reed-Muller codes. In 1970, Delsarte, Goethals and MacWilliams gave a %characterization of minimum weight codewords of Reed-Muller codes and also formula for the minimum weight codewords of Reed-Muller codes. Carvalho and Neumann in 2020 considered affine Cartesian codes in a special setting where the subsets in the Cartesian product are nested subfields of the given finite field, and gave a characterization of their minimum weight codewords. We use this to give an explicit formula for the number of minimum weight codewords of affine Cartesian codes in the case of nested subfields. This is seen to unify the known formulas for the number of minimum weight codewords of Reed-Solomon codes and Reed-Muller codes.