One challenge is that “plenty of space” is relative. How much free space does your Mac say it has?
Some Mac users think 20GB free is plenty, and for some kinds of non-graphics work you can drive free space down that far and the Mac still runs OK. But pro graphics applications (photo, video, audio) often make heavy use of large temporary files to take a load off of working memory, so having 100GB or more of free space is more reasonable for those types of applications.
Also, Photoshop often disagrees with macOS about how much free space there is, partly because Photoshop does not appear to count up free space the same way APFS (your Mac’s file system) does, and the way APFS counts free space is confusing anyway. To get past that often means freeing up more space than you think you need.
The size of the Photoshop document affects how much scratch space you should think you need. Small web graphics might be editable with only 20–50GB space available on the scratch volume. But working on large images at print resolution, especially at higher bit depths with lots of layers and masks, may sometimes require 100–500GB of space on the assigned scratch volume. My MacBook Pro doesn’t have that much free space, so I attach an external empty SSD and assign that as my Photoshop scratch volume, and so it’s fine.