Sulfide contains 18 electrons. It is a negatively charged ion of sulphur, and its charge comes from having two more electrons than common sulphur atoms.
Ions are formed when an atom of an element gains or loses electrons, creating a mismatch between the number of electrons and the number of protons. Since electrons carry a negative charge, having more electrons than usual gives an atom an overall negative charge. Sulfide's charge makes it a strong base, as it can easily recombine with hydrogen to take in a proton and balance its electronegativity. Bases in general are molecules that easily donate electrons, and sulfide has two to offer, making it rather reactive.