Mac Mini M4 16 or 24gb ram?

16GB might be enough if you work on documents of relatively low width and height (in pixels) that don’t have many layers. The Photoshop system requirements might say that 8GB is the minimum, but that’s just to get the software started. For frustration-free work, 16GB is considered rather minimal today. Especially after Apple introduced Apple Intelligence, which is said to make the OS need a little more memory, leaving less for applications. And also because an Apple Silicon Mac needs to use at part of system memory for graphics acceleration.

For 11 x 17 inches at 300 ppi, 24GB should be OK. 32GB or more might be better if you tend to use many layers, or like to keep many other apps open. Also make sure you have internal or external storage set aside for the Photoshop scratch file.

If you want to make sure, here’s what you should do: You currently have a Mac with 40GB. The next time you do a digital painting, open Activity Monitor (it’s on every Mac), and switch it to the Memory tab. As you work, watch the Memory Pressure graph at the bottom of the Activity Monitor window.

If Memory Pressure is always green and very low, then your current 40GB is more than you need, and 24GB is probably fine. 

If Memory Pressure is usually green but occasionally yellow, then you probably need at least 32GB. 

If Memory Pressure frequently pops up into the orange or red, then your current 40GB is already not enough and the next Mac should have 64GB.

What you need to keep in mind is that Apple silicon uses shared system memory for the GPU - and the GPU can eat up a lot. So the basic rule of thumb should be to double the amount you would normally need.

So no, 16 will certainly not be enough. 32 should be absolute minimum, 64 more comfortable.

But that's only half the story. Photoshop's total memory requirement exceeds that by orders of magnitude. There is no such thing as "enough RAM". So temporary working data are written to disk, known as the scratch disk. Think of the scratch disk as Photoshop's main memory, with RAM as a cache holding the most current data. You will need between 200 - 500 GB free space for the scratch disk, depending on file sizes, how many open files, and number of history states.

In practical use, the scratch disk is more critical than RAM (as long as you have the base minimum).