Can anyone tell me how much buttercream I will need to fill and Ice 6,10,14 round cakes plus ice a 11x15 sheet cake. Also how much filling do I need for the 11x15? How many cups of icing shortening is in a pound? It seems to vary at the cake store I purchase it from so I would like to know what amount I'm actually supposed to be getting. Thank you

this handy chart has icing amounts -- 

http://www.wilton.com/wedding-cake-guide/cms-wedding-cake-data.html

depends on how much filling you want to put there -- i try to use as much filling as possible but some peeps use filling like mayonnaise -- 

just average out the cup measurements -- according to google a pound of shortening is about 2.2 cups