what if i use buttermilk instead of sour cream? and how much?

I almost always sub buttermilk for sour cream. It's a trick I learned from my mom. Yields a just-as-tasty result with less fat. Measure one for one.

You can sub one for the other. I would use the same amount of buttermilk as sour cream. The resulting muffins won't be quite as rich, especially if your buttermilk is low fat or fat free, but it will work. You could also use whole milk yogurt.