“How many sides does a standard stop sign have?” — A standard stop sign has eight sides.
Its eight-sided shape makes the sign an octagon. In the United States and many other places, the distinctive outline lets drivers recognize a stop sign even when they see its back, at night, or from an angle where the lettering is unreadable.
The shape became a major part of American sign standardization in 1922, when the American Association of State Highway Officials selected the octagon for stop signs. Early signs did not always use today’s familiar red-and-white design; older versions could have different colors and proportions.
The octagon is widespread but not universal worldwide. International rules permit other designs, and Japan uses an inverted red triangle. That is why “standard” matters in this question: the conventional stop sign familiar across North America is an eight-sided red octagon.