Is a pattern discovered by the American scientist-psychologist George Miller. The law states that the average person can only keep 7 (plus or minus 2) items in their working memory. Miller's law for UX is about the average person can retain, give or take, seven pieces of information, seven items from a group of information in one sitting.
In 1956 Miller asserted that the span of immediate memory and absolute judgment were both limited to around seven pieces of information. The point where confusion creates an incorrect judgment is considered channel capacity.
Lots of designers are convinced that limiting the number of menu tabs or items in a dropdown list to 7 is a false constraint. On a webpage, the information is in front of users' eyes, they don't need to remember anything, and so they can easily manage a wider choosing. If you hide some of the menu items, you can make it harder to access the functions and prevent people from learning about them.
If there are fifty items in the menu, then, of course, to display them as a solid sheet will be bad for perception. But if you leave 7 of them, and hide the rest behind the dropdown this will not make life easier for the user, but only complicate it. The design goal is to find a way to organize these fifty items so that they can be understood — for example, to group them.