If you were going to design a bipedal humanoid with tool-using capability (not sure if those are exclusive to each other) how many fingers/toes would you give it? Is there some advantage in gripping things with 5 as opposed to 3?
I’m not suggesting any kind of “intelligent design” idea here, btw, just wondering if aliens with more or less would have some kind of an edge.
5 is nice because it has some built in spares (people who lose one or two fingers can still learn to function pretty well without them) without being too wide to grasp the other things that have evolved on our planet, like nuts and berries and remote controls. 
I think the only design change I might make is to try out gliding joints (like our thumbs have) on more than one digit, maybe on all of them. I’m not sure if it would make a huge difference, but it would make losing a thumb less problematic.
Oh, and I’d make at least two of a woman’s fingers long enough to reach her cervix. Mine just aren’t, and it’s a bummer for some forms of contraception.
Five fingers is okay, but I’d probably replace the pinky finger with another opposable thumb. And make the toes prehensile, of course.
This paper may yield some idea. (If it does let us know!) I think one point it makes is that once the 5-size was settled (randomly(?) millions of years ago), the evolution 5–>4 was much more likely than 5–>6.
Wouldn’t that make it harder to walk? The joints and all would be less able to take weight and pressure from walking, wouldn’t they?
It would take significant changes to shoe design, for sure. But even now the weight isn’t carried by the toes. When you “walk on your tippy toes,” you’re still putting the weight on the balls of your feet. I presume that would still be the case.
There’s no obvious reason the fingers & toes have to be the same number. Even if they started out that way back when we were all some variety of tadpole, there’s no reason they have to stay that way.
Our current design for great toe certainly isn’t opposable to the same degree the thumb is. So there’s no reason a future opposable pinky finger would demand a similar change to the 5th toe.