It depends. If you are prone to forget something, three is a safety number. One at home, one in your bag and the third in your office. In that case, you will never ever lost sunglasses.
But in my opinion, sunglasses is not only a weapon protected our eyes, but an important accessory as bag, scarf. To comply with your dress and makeup, you need at least two pairs with different color or material. Otherwise, sunglasses can decorate your face size.
That depends on your hobby, economic condition and so on. If you are a fashionable man, you can choose several types of sunglasses with different colors, designs, materials and so on. However, the sunglasses are mainly used to prevent from the uv and other strong sunlight for the eyes. One or two types of sunglasses is suitable for you.
Actually, it depends your habit. In my opinion, as many pairs as possible. Since the sunglasses nowadays are either protective necessity or fashion icon, I think you need to have at least two pairs. However, if you are really a forgetful person, you can leave each place where you stay a spare pair at hand. Hope this helpful.