How many whetstones, and what different grits, do I need? My grandparents never altered their WÜSTHOF 4596-7/20 Classic Ikon 8-Inch Cook's Knife since buying it on Aug 1 2018. It could effortlessly slash coconut and raw Winter Squashes, but now it can't even cut paper or pears! Undeniably they must alter their knife.
This website counsels:

Low grit sharpening stones are also great for a knife that is very dull. If your knife isn’t able to cut paper at all then we’d recommend starting with a low grit.


This counsels

The 1000 grit stone is considered your basic, go to, sharpening stone. If your knives have lost their edge and need a good sharpen, then this is the grit you should start with.
[...]
You shouldn’t use this stone often, as it will wear your knife down. The 2000 and 3000 grit stones can be used more often if you are the sort of person who likes to sharpen a bit more regularly as they are less coarse, but again, they are designed for sharpening and not maintaining your edge.

So my grandparents are baffled. Ought they buy 1 1000 Grit whetstone? Or 2: 1 at 500 Grit + 1 at 3000 Grit?
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.