The number of breakpoints a website should have would depend on the nature of the website. If the page simply contains nothing but text which is intended to fit the entire width of the page, there might not be much to gain from setting up several breakpoints. But if the page contains images and other visual elements including boxes with content, etc., size would be one consideration, but so might the positioning of some elements relative to other elements… above, below, side by side, etc., and it might also be important to prevent some things from displaying at all in some cases.