How many whetstones, and what different grits, do I need?
Several companies sell two-sided whetstones or waterstones are 1000/3000 grit or 1000/6000 grit (this King waterstone is the one I have), or similar medium/high grit combos. You want one of these two-siders; it's a nice compact way to get the sharpening you really need. These are what I recommend for a home use stone.

However, if your grandparents aren't used to sharpening knives at all, please reconsider giving them a whetstone/waterstone. Sharpening properly using a stone takes a bit of technique and self-education (I took a 2-hour lesson), and someone who isn't into knives is liable to either damage their blades, injure themselves, or both. Some kind of sharpener that guides the blade would probably be better for that kind of a cook.