250GB is not enough for Photoshop on Mac? It keeps saying the disk is full.

Indeed. My general advice is 250 GB minimum for light and occasional work, but for serious work you should have 500 GB to a TB or more. Scratch files are orders of magnitude bigger than nominal file sizes, for the reasons Dave listed.

If you're stuck with the disk space you have, and you need to have that many files open, then reduce history states to somewhere around 5 to 10. The default number of 50 states is a huge drain on disk space.

The scratch disk contains the working data for every open photo, it is used as temporary working memory. A 50MB image file is compressed when saved, but when opened needs much more memory than that (3 bytes for every pixel on every layer in 8 bit, 6 bytes if using 16 bit, plus all the history states, the content of smart objects ...etc). Content of the scratch disk is then constantly swapped with RAM as you work. Scratch disk space can therefore grow huge and what you are experiencing is not unusual with that many images open. If you need to work with large images, or large numbers of open images, then you need large available scratch disk space. Here I use two disks, the first with 900GB free and the second with 700GB free. Neither are my operating system disk.

Dave