Script for paint by numbers

Can you change a photographic (or painted) image into a paint-by-numbers-image that meets your expectations (disregarding the numbering, so just focusing on the isolation of pertinent areas) without manually painting, creating paths, …? 

Is this possible, is this difficult?

I would estimate that it is possible.

Whether it would be difficult for you seems difficult to assess – what is your Photoshop Scripting experience?

Whether the resulting images would match your expectations is yet another question. (Edit: A »classic« automated approach might not accentuate the details you would find most relevant.)

https://community.adobe.com/t5/photoshop-ecosystem-discussions/assigning-number-to-color/m-p/11803454

You could make an action using the steps below to reduce the number of colors and have Photoshop outline the areas where a color goes.

1. If you open an image that is on the Background layer, click the lock icon on the layer to make it a regular layer.
2. At the bottom of the Layers panel, click on the middle icon to apply the adjustment layer Posterize.
3. Set Posterize to the number of colors you want in the image.
4. Optional: if you want to change the number of colors later or make other changes, you can select the image and the adjustment layer, right-click, and Convert to Smart Object.
5. Go to Filter > Stylize > Find Edges to outline the areas of color.
6. Optional: if you want to make the outlines grayscale, you can add a new adjustment layer for Black and White.