PLEASE can someone help. i need to know how many batches of butter cream to make to cover a full sheet cake

This link is to Wilton's serving/batter/baking time guide. In it you will find the approx amounts of icing you will need to ice and decorate a cake. The various sheet sizes are listed as well.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

and here is a link to a thread that has frequently used acronyms on CC (great for newbies and oldies alike):
http://www.cakecentral.com/cake-decorating-ftopict-2926.html

Hi and Welcome to CC, ladybugpatg.

Just what xstitcher said!

Also, if you're looking for tasty but high performance crusting American buttercream recipes....

Everything you need to know to make, decorate and assemble tiered/stacked/layer cakes:

http://cakecentral.com/cake-decorating-ftopict-605188-.html

HTH