This has not been talked about yet, but it is important to understand. While the number you will need will vary on a few factors, you will need more than one. There are even situations where you will need upward of 20 attached to your harness. Now, this does depend on the length of your climb and where it is taking place.
If you are thinking about an extra-long climb, then you may need yourself 20 or more. And, yes, this is where the weight also becomes a significant concern.