When people give something a number rating, they usually have an idea of what the top possible rating is. For example, you might rate something a "6" with the idea that the top possible rating is "10". That top value defines the "scale" of the rating. So you can say:

I'd rate it 6 on a scale of 10.

or

On a scale of 1 to 10, I'd rate it a 6.

The most common scales are one to three, one to four, one to five, one to 10, and one to 100.