You will too need three, if you want to retroreflect a laser.
The right answer! =)
Oh, I dunno. I think it only works if you are skinny. I think the more amply figured of us may need an extra mirror or two to peruse various aspects of our buttockal regions.
I suspected I was totally off. 
Wait? That’s really the right answer? So why can’t you substitute “in” for “for”?
How many mirrors do you have in your back?
Don’t know. I don’t have a mirror to check back there.
But it’s not in YOUR back. The riddle says in THE back. You can have a mirror in the front and in the back, no problemo.
But “Two (mirrors) in the back” wouldn’t let you see anything. It’s two mirrors for the task of seeing your own back.
I am SO glad this thread has a “real” answer - just when you think you’ve forgotten about the nightmare that was “14 k of g in a f p d”, something pops up to remind you. Oh, and I may as well declare I’ve just lost the game, while I’m here. You can all send your thanks to the usual address.
I use three mirrors to cut my hair. The one in the front, and one on each side (these two are hinged so I can see the back of my head and cut straight). Technically, I use all three, but only use the side two in concert to see the back. I only need one to cut the top and sides.
The tailor I use has a three mirror set so I can see what adjustments he’s making to the seat of my pants and the back of the pants cuffs, so I guess that’s three for the back.
I was thinking propellers. There’s sometimes one in front, and sometimes there’s two in the back without the one in the front, like the A-10 Warthog. (I know the A-10 has turbines instead of props, but I’m sure there’s some WWII plane that has two in the back.) Sometimes they have four, but I don’t think I’ve ever seen a plane with three props.
Never heard of a Ford Tri-Motor? Or a Ju-52? Now I grant you that the Flying Shoe was an oddity, but the Italian airforce had several three-engined bombers, whereas I’m struggling to think of any twin-prop pushers outside of Raiders of the Lost Ark for this timeframe; twin-prop planes have overwhelmingly been tractor configuration, and where pushers have been used at all, they are almost invariably single-engined.
I was thinking of the letter I in US state names. Sometimes one for the front (Indiana etc.), sometimes two for the back (Hawaii), but never three (consecutively). But it fails the substitution test - you could just as easily, and probably would, say at the front.
I guess all those department stores with 3 way mirrors are fools.
Easier, more convenient with 3 or more? Sure, but not necessary.