How much white tint do I use to for 8 cups of Buttercream?

I don't believe that adding white food coloring (which is what I'm assuming you mean by white tint) to buttercream will give you white buttercream. I'm also assuming that your buttercream has a slight yellow appearance in it from being made with butter. Adding a small amount of purple (a very tiny amount.....like a flat pinhead size) should counteract the yellow color and make your buttercream white. A word of caution....add too much purple and you will get lavendar icing. I would do a small amount first to get a feel for how much purple is used.

~Chelle