Dealing with a hank of natural sausage casings
Hanks of casing aren't generally that big (of course, this is relative - if you're only making one pound of sausage it's gigantic!) so I've always soaked the whole thing.  They should be slightly easier to pull apart, but it helps a lot if you soak them in slightly warm water and then after they've soaked for a bit, find the end and run warm water through the hole (so it goes into the casing), then pull the end out of the hank so the water runs down and through the inside of the remainder of the casing.  The water will bulge out the casings and help to make them easier to remove and also wash out the inside (which also contains some salt) - once you have the casings separated as you want them, you should wash them out using this method again.
I have saved the leftovers in the past, but how you do it depends on how long it will be before they're used. In any case, you should rinse them in warm salt water as when you're making sausage, bits of meat tend to find their way where you don't want them.      
If you're going to use them soon, just make a more heavily salted brine than what you rinsed them in, make sure the brine covers them completely and there's no air inside, and put them in the fridge (ideally in a dark container)  If it will be longer than a week, you can brine and then freeze them - but this is a bit of a controversial issue.  Some say this weakens the casings, but I've done it in the past for up to a month with no problems.  Your milage may vary.  In the future, if possible, only buy what you need.