Well, you didn't say what cruise you were doing....WDW first or not?
For 2 of us on a 7 day, we usually take 3 suitcases total. I pack one for each of us and a "cruise only" bag of our dress clothes and other things that we won't need till we are on the ship.
Hints for storing luggage--after the safety drill, place the life vests under the bed. This frees up a large closet shelf that will hold suitcases. Most cases will slide under the bed (you might have to lift the bed a little to get past the lower railing.
The airlines generally allow 2 bags plus a carry on. Your biggest problem will be what to do with all the clothes, etc. in the cabin. We store some of it in the suitcases (snorkel equipment, etc.) Also, the shoe holder on the bathroom door is a great trick for organizing everything. If you have the split bathroom (cat 10 or better), you'll want 2 of these!!! We use them for toiletries, camera, kids KTTW cards, autograph books, sunscreen, etc. SOmeday, we might even use them for shoes!