I'm a size 10, what size fits best?

I bought the 6-10 with a 10-10.5 shoe size. Wearing the socks for any period of time was borderline painful and very tight. The socks lasted about 4 months before all of them had holes poking through the big toe. Recently reordered but got the large (11-13) size instead and they are comfortable and fit fine. I would probably get the large even if you were a 9 shoe size. Unless foot-binding is your thing of course.

I recieved the socks and got 4 pairs . Is it 8 pieces 4 pairs , or 8 pairs?

It should be 8 pairs ( 2 sets of 4 different colors) 16 pieces total.

What percent of cotton?

These are the WORST socks I've ever purchased after having bought hundreds of socks online. Don't be fooled, as I was, by the cotton percentage. They are so thin the bottoms wore out the first day, they are way too small, elastic is too tight, there is zero texture that could at all be called "cushioning", as they describe, on the bottom. Like wearing kleenex. They're very uncomfortable and not even putting on.

If I wear a 8.5 men's shoe what size sock should I wear

I wear a 10 and ordered the 6-10 size. They’d probably work for you - but are insanely small on me. Offered a review to the seller but never heard a word back.