I hog with 50 or 60, clean up with 160 Gators and finish grind with 400 Gators (all from Tru-Grit, available there in 2x42). The Gators are worth every penny, they cut well and last a long time. The light touch is very important on such a fast machine.
Some will tell you that you can't get a decent finish unless you use every single grit belt known to man in increments of 10, but that's hogwash. If you want a nice satin or high polish you're going to be doing some hand-sanding to get the 400-grit scratches off anyway. When you get to hand-sanding it does pay to step up through the grits, I go 400-600-800-1000-1500-2000. It goes faster once you get things nice and clean with 400 - don't rush through that part!
Quantity depends on how many/how big a knife you want to make. Order as many as you can afford, say 5 of each, you'll soon find which ones you eat up the fastest. Get a belt cleaner stick (looks like a big tan eraser), they do help your belts last a bit longer and prevent nasty scratches caused by broken grit and swarf in the belt.
Incidentally I've ordered several times from TruGrit even though I'm in the States... I'm quite pleased with their service.