Most sharks have five gill slits on each side. These openings let water leave the throat after it has passed over the gills, where oxygen is extracted.
Sharks do not have the bony gill cover, or operculum, found in most bony fish. Instead, water enters through the mouth and flows across the gill filaments before exiting through separate slits. Some sharks maintain this flow by swimming, while others can use movements of the mouth and throat to ventilate their gills.
Five gill slits is the standard pattern, but it is not universal. A few shark groups have six or seven pairs. Sixgill sharks and sevengill sharks are notable examples, which is why the wording “most sharks” matters in this question.
Another structure can cause confusion: the spiracle is a small opening behind the eye in many sharks. It helps draw in water, especially in bottom-dwelling species, but it is not counted as one of the visible gill slits. Rays also commonly have five pairs of gill openings.