I've done a king size using as little as 5 bobbins for my Handiquilter, and I've done others that I've used 11 bobbins. I've gotten pretty good with estimating what I'll need, and wind a bunch before I start quilting.
When I first started with the Bailey, I bought a bunch of spools of cotton thread from Connecting Threads - 1200 yards, and I'd often use more than one to finish the quilt, top and bottom thread. Don't think I ever used up two though. The Bailey loved the stuff, so I bought a large selection, 2 spools of each color. Then moved to the HQ and it hates that thread.