I like pattern mixing and have tried to figure this out since we bought our house and I got into decorating. Here are a couple of helpful graphics I've saved over the years.

Some principles are hard and fast and some are suggestions.

It's a good rule to have a solid color pillow, a small-scale patterned pillow, and a large-scale pattered pillow. That makes things interesting.

Instead of looking for a formula or hard and fast rule, ( there are none ) why not post what you are trying to accomplish? Specific to YOUR OWN space?

I think both design principals are valid. It's also OK to not have patterns or prints in your fabrics if you don't want them. Lots of folks have solid colors without patterns to them. Some folks like tons of textures and patterns and really mix it up.

Depends on ones personal style and needs. What is valid for one person might be invalid for another.

No right or wrong, base on how comfortable you are picking/mixing and the outcome you want. Best method is to start with the rug (that suits seating and wall color), bring home lots of pillows to test and neutral drapes if you want a calmer effect (and will be easier).

I have never liked mixing too many patterns so Ian prone to very few in my spaces , sutle ones like a rug can sometimes be where I use a apttern . IMO deisign rules are a guide not written in stone. Maybe the reason is we collect original art so that is usually my pattern.