I've never run it solo! The conversation of it all is so important to me, BUT I think... 3 paints 3 touchstones feels good. I'd maybe do:
the Paints as write 4-6 and randomly grab 3-4 of them, to simulate a little "out-of-your-hands" inspiration. OR write 2-3 Paints and close your eyes and randomly point to a word in the suggested Paints once or twice lol
And with Touchstones, I like to have a Touchstone for each that adds a: Thematic Element, Aesthetic Element, Sensory Element, and/or Tangible Element (you can see this in the example Touchstones in the book!)
Then after that, yeah if you're comfortable with answering prompts and riffing off yourself, then you can absolutely play solo!