How many electrons would be neede to equal the mass of one neutron

A large amount. Neutrons have a mass of 1, but electrons' masses are almost nothing.

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.