If its your first plunge into the sausage world by all means talk to your butcher. BUT realize you can buy a hank from a commercial vender sizablly cheaper. Thats where the butcher bought theirs.
I bought my first "Bag" from the butcher to do chicken sausages (yuck, btw), after that it was Butcher and Packers.
I have 5 or 6 packages (hanks of different sizes now) I just keep in the reefer. Some are over a year old and still work fine.
Someone above said something about boykjo's casing tutorial, I highly recommend it. After you live thru the local butchers leftover bag, you'll be surprized how easy a hank is to handle when you know how. Read the threads, when you buy a hank know what you intend to do, and what will be your most common, then buy accordingly. I use 46MM most common, but few others have even had a need for that.
Believe me you think you'll never use up a whole hank but............
BTW DigginDogFarm, Martin somewhere here posted a great set of charts to calculate approximate capacity per per size per hank. Its a great resource.