Hi Chris,
We filled our cookie cutters to the top so they would felt down to be about 1/4" thick when done. I'd say about 3-4 layers -- that completely fill the entire cookie cutter -- it's better to be too thick, than too thin! I'd estimate maybe 3.75" X 3-4" of white for each felted piece should do it. If you're going to add pink and black you'll just need a little piece for each felted sheep -- about 1-2 inches should do.
Good luck! Post pictures when you're done!
Blessings,
Jaymie