well i have about 30 hanging on the wall from 50 grit to 600 grit, you should be able to make a knife with 1-50-80 grit and 2 -220 grit 1 -400 grit, it depends on the finsh you want. alot of hand sanding goes with making a knife. these belts should last awhile, one thing to buy is a belt cleaner, if you don't you will go through a lot of belts.
jump in, you can do it
good luck, be sure to post pictures of you first one.
dana