macos big sur storage

You will need a whole lot more than that. At a minimum, it requires 35 GB of space for the download and install. If you are on earlier OS versions, it will require 45 GB of space.

Additionally, you should always maintain an absolute minimum of 10 - 15 GB of empty space for the OS to function properly. Aside from storage space, your disk also contains the OS and system files which need room to function properly, especially on an SSD for its garbage collection process. If you allow it to get close to full or completely full, your computer will simply stop responding or functioning. At that point, it will be almost impossible to rescue or move off some files.

Well, you can, or you can move some space hogging stuff (videos, music, photos, etc) to an external hard drive. Or delete what you do not need.