You only really need one sample of a rock, but it always helps to have more. If you only have one sample, you will just have to research the rock on google. Wikipedia is also really helpful, because they usually have articles that outline every variation. Plus, the rocks that you will get grilled on the most (fluorite, calcite, etc.) generally have long articles, because they're complicated or very common.What about the variations on color / habit of a rock though? Would you have to get multiple samples for each variant, or is just one sufficient.
What traits do you guys normally use for ID?
For ID, I use crystal structure the most, followed by texture, and then density. It also helps to know what shades of colors they are (dark vs. light), and their translucency. Some you can tell automatically just from one or two traits. Galena (the heaviest for its size,) for instance, or graphite (rubs off on your skin).
On the side, I think next year they might be introducing the history of geologic discovery to the possible topics. The nationals test had some questions about Mohs that I think most people weren't expecting. We had the answer in our field guide, but that doesn't always happen. I wouldn't count on it, but if you have some extra time, go Wikipedia hopping and get a paragraph or so on the major discoveries and geologists.