In my peacock scarf I have a 8/2 cotton warp and a 20/2 bombyx silk. How do I wash this and will it Ruch?

I don't believe it will ruch, as the cotton will only shrink about 15%. Generally for ruching to happen you need a non shrinking fibre such as the silk with a fibre that can be fulled, like wool or wool/silk or alpaca. Cotton does not full. And the shrinking and non-shrinking fibres need to be in stripes in the warp.

When it comes to a project containing both cotton and silk, you will want to wash as if it was 100% silk, so gently by hand, in warm water and with gentle soap, rinse, and lay flat to dry. Don't wring it out, that could introduce wrinkles. Press when almost dry.

The combination of 8/2 warp and 20/2 silk weft is delightful, I think you will be happy with the finished fabric.