Megapixels measure the total number of pixels, not the shape. A long, narrow banner and a square can have the same count. Cameras are sold by this number, and a current phone records far more pixels than a product page ever displays.
A few marketplaces limit uploads by megapixels in addition to, or instead of, a maximum width and height. A photo straight from a phone can exceed such a limit, and so can a heavily upscaled image. The remedy is to export at smaller dimensions; the marketplace would scale the picture down anyway.
More megapixels do not mean a better product image. Sharp focus, even light and a product that fills the frame matter more, and beyond the size at which zoom works the extra pixels only add weight to the file.