If we're being realistic, five knives would do everything we need. It's important to remember that a knife is part of a larger tool set, and we forget that in the midst of discussing the difference between face-centered carbide and body-centered carbide, or the effect of Martensitic inclusions on shear failures in knife blades. Knives are for cutting. The rest is personal preference, and to some extent, other people's perception, which brings us to an important philosophical consideration, regarding Swiss Army Knives.
I believe the difference between a SAK/multitool and a knife, is an important one. I'm an advocate for responsible tool use, which includes the delicate people who believe that any knife outside the kitchen is a "dangerous weapon" (my Boker Nano, for example. True story). Regardless of how I feel about the person who is having a sensitive moment about my folder, part of my responsibility is to show them that knives cut things, full stop. The "good" or "bad" that they are seeing is because of what the knife user chooses to do. I don't expect to change their minds, but I want to make them work hard to justify their pre-conceived notions.
The products of Wenger, Victorinox, Leatherman and the rest, don't seem to get covered by this perception, and sometimes, it's easier to carry a MT than wear my metaphorical lapel pin, proclaiming that I'm a Repsonsible Tool User. Or, just a tool. Depends on whose watching. That's why my "wedding & funeral" carry is a Skeletool CX: I don't have to worry about being some version of "that guy", if I need to use it.
I always throw a bunch of footnotes at threads like this, anyway. Gifts, especially from my kid, don't count. I'll keep those forever, even if they don't make it into the rotation. But if you think about it, those aren't really knives, they're a tangible focus for your feelings about another person, so their identity as things is not really important: it could be a painted rock, and it would mean the same thing (yes, I have a painted rock).
One of my other footnotes is about machetes and other long-bladed implements: they're not knives. It's like driving an M35 as your daily: you can, and in some places, it makes sense. It's a purpose-built item, and if somebody asks me to choose five vehicles to put in my hypothetical garage, the Deuce might be one of them. But it wouldn't be the everyday choice.
I believe that for “individual” set it’s need to be at least 5 knives:
