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.

I agree with the two sided recommendations for whetstones as a nice compromise, and no need to try to cover the full professional range. One for repair and one for finish is normally adequate.

However, I would recommend not starting with that route at all. You have a relatively expensive knife that you describe and needing a new edge. I would leave that to a professional and pay to get it re-edged. Attempting to do that when not familiar and practiced with a stones is likely to lead to buying a new knife. A professional can get your knife nicely tuned up to the correct edge geometry and sharpness. At that point, care and regular honing will mostly keep it there and you can then use stones as needed on an edge that is starting closer to correct. If not familiar with stones, practicing on lesser blades is invaluable learning. I ruined several cheap junk knives learning to get angles correct and smooth without rounding. The cost of getting the knife back to original or better condition is cheap compared to replacing it, and then you have a good base to maintain.

As already mentioned by FuzzyChef, you can buy two-sided whetstones/waterstones from many companies. I myself have a 1000/3000 Wüsthof waterstone at home that I use approximately every 2 months or so, and a honing steel inbetween.

I would consider a 1000 grit stone to already be a "repair stone" for a really dull knive that does more cleaving than slicing. Since you mentioned the knife is not that old, I think only a little bit on a 1000 grit followed by a 3000 grit would already sharpen it perfectly for a home cook.

However: doing this takes skill. Doing it improperly will damage the knife more than help it. If your grandparents aren't skilled, or do not have the fine motor skills due to age, etc. I would consider buying a handheld "pull-through" sharpener for them. Yes, these are not ideal. Yes, they might take away too much material over time. But they're still better than using a dull knife.

To write an answer that is not an alternative suggestion: it's entirely up to your personal preference.

First, you will have a preference for the knife sharpness. If you don't mind having a dull knife, the answer is zero. If you want a knife that is sharpened to a certain finishing quality, then the grout of the finest done you buy is determined by the quality you want to sharpen to. It's quite trivial: if you insist that your knife is as sharp as a 4000 can get it, your finest stone has to be 4000.

The sharpness of your lowest grit is determined by the frequency with which you want to sharpen your knives, by the way you use them, and by your sharpening skill. If you frequently cut hard things (like animal bones) or use hard cutting boards (e.g. glass), your knives will get larger nicks and they will need rougher stones. Also, with the usage parameter held constant, if you sharpen the knives rarely, they will get duller between sharpenings, and you'll need lower grits. Your skill means that you risk damaging the knife when making a mistake, and lower grits will lead to worse damage. (Although once damaged, it is easier to repair with the coarser stone, do this one kinda evens out).

The in-between grits depend on a trade-off of time versus money and storage space. In theory, you could try sharpening a very dull knife with a 8000 stone only. In practice, you'll sit there several days and probably give up before you're done. It's quicker to get from your lowest to your highest grit by using many grits in-between, than to try to achieve the same result with fewer intermediate grits. So, if you buy more stones, you'll work quicker (up to a point, of course - imagine what it would be of you were doing one stroke on grit 451, the next on grit 452...)

So, in the end, you cannot know how many grits you need, and which ones, before you have spend some time sharpening and observing your preferences. The typical way to solve this catch-22 for nonrentable items like whetstones is to buy a cheap mid-range one, use it for a time, then reflect what is bothering you and which stone could be used to address the problem.

I concur with the previous answers that the best in this case may be to leave it to a professional.

However, if you want to do it yourself, another option is to use 'diamond cards' rather than whetstones. Whetstones tend to be heavy and fragile, and will eventually become concave, whereas diamond cards are small and sturdy, and will stay flat (but they may lose some of their 'bite').

The biggest brand is DMT, but you can find cheaper copies for not very much on-line.

Something else to consider is a guided sharpening device. I'm not talking about electric sharpeners, which are mostly junk and could damage your knife. I mean something like an EdgePro. If you don't have experience free-handing the angle while sharpening an expensive knife, as others have noted, you can do more damage to your knife and make things worse. I'm not necessarily recommending a specific product (I've seen other systems other there), but the idea is something that will allow you to set an angle precisely to allow you to sharpen correctly.

While I wouldn't start on your most expensive knife, with a device with guides like an EdgePro, you can practice on a cheap knife and easily get experience to sharpen an expensive knife with the precise blade angle(s) you want. (Of course, that device may be too expensive for the one knife you're talking about. But for someone with a larger collection of knives -- or relatives who also need sharpening -- and without a lot of sharpening experience, such a device can help guide you and ensure you don't do damage.)

As for grit recommendations, most other answers cover the basics. I'd just add that it's important to realize not all companies' grit ratings are the same. I've definitely encountered a stone from one place that was listed as 500 grit which was more like a 1000 grit stone from another company in terms of roughness. It's tough to know this when starting out, so I'd tend to get a recommendation from someone who actually knows the specific stones and equipment as to what would be a good starter set.