What is the relationship between the 6s in he number 6,647?

I'm not sure if you mean 6,647 as a decimal or as 6 thousand 647. I'll do it for both:

In a decimal of 6.647 the first six is for ones and the second six is for tenths, which means the first is 10 times as much as the second.

In a number 6647 the first six is for thousands and the second six is for hundreds. It also means the first is 10 times as much as the second.