Some people do have 6.

Why 6??? Why not 5???

But seriously, let me make the case for 5.

To grip anything you need at least two. But two digits will give you an unstable pincer-like grip so let’s call 3 the true minimum. So we need a minimum of 3 but our control and precision is improved with another one. And it’s a good idea to have a backup in case one gets injured, so that brings us to 5. Five is more than we honestly need and evolution budgets very strictly, which is why you already see finger #5 shrinking in size so it costs less resources over your lifetime. It truly is there just as a backup.

Now forget all that because it’s bullshit. 5 digits was established in our ancestor organisms before gripping things even came about with the opposable thumb. So what were those organisms doing with digits? Walking on them. There are two styles of walking tool: paw and hoof. The paw has flexible digits that help mold the step to the shape of the ground for maximum stability and traction. As a bonus you also get some sensory feel. Hoofs go the other way: a hard and durable shoe that will compress the soft ground or catch on hard surfaces. Organisms that run fast and far tend to have hooves because they aren’t picking their path super carefully and durability is the most important thing. Predators tend to have paws because they ARE picking their paths with great precision and they need to walk carefully, sometimes with stealth, and be able to build up their traction for a spring.

So if you’re going to design a paw with flexible digits that can mold to the ground, how many digits should it have? A single digit is no better than a hoof. Two points on the ground form a line which is tippy and still not stable yet. You need the third minimally to form a triangle that’s stable. Now control and flexibility to match uneven ground surface will increase with a fourth. And it makes sense to have a backup in case one digit is damaged, so you have five. However, fine control isn’t as important for paws as our hands, so the added value of fine control and having a backup adds up to a less, and evolution budgets very strictly, so you see digit number 5 fading from pawed organisms, and already vestigial in many. You could consider the “palm” of the paw to be a digit that works in tandem with the others, though, which gets us back to 5. Five really does seem to be an inevitable number, doesn’t it.

That should cover it. Be sure to drink water before bed.