No matter if you have just a few raw materials and suppliers or several hundred, I think you need to track what's important to you. What are the factors that are going to ensure the success of the purchasing process?
You might want to look at nonconformances found at receiving inspection to determine if you are getting good raw materials. If the raw material arriving on time is important, you should measure whether suppliers deliver by the expected due date. If you are experiencing errors like the ones we were dealing with, you need to make sure accurate information is being passed on to the people doing the purchasing.
In short, there is no one answer for every organization. You need to determine what matters for your success and start tracking that. I would suggest starting simple with just one (or a few at most) key indicator and build from there. If you start with a whole mess of numbers, it will be harder to determine which are really the best indicators and your management may drown in all the data!
MrMiagi's comments are along the same line and makes the excellent point of passing the information along to your suppliers.
Hope this helps!