It’'s an onion ring that goes all the way to infinity, obviously.
Did I tell you about Lady Madonna trying to make ends meet, looking through a glass onion-ring?
It took me eight minutes to read this thread? Damn.
IIRC, my guitar player’s fender amp goes up to 12. He usually plays it at about 1.8.
As Biffy already correctly pointed out, it’s The Infinite Onion Ring.
Check the very end of this Mastercard ad:
Please say you’ve told him “We’re a little too close to the edge of the cliff. Put it down to eleven.”
Yeah, I used to have a Fender amp that went to twelve. It’s a bit of a waste if he’s playing it on such a low setting. Guitar amps often sound best when turned up (not loud, just turned up, I’m suggesting he should have a smaller amp rather than turning the current one up more.)
If you ever want to discuss amps and how to find their sweet spots, that is an area of particular geekery for me, but we digress. But yes, as I stated before, many amps, including Fenders, often have knobs that go to different numbers than 10.
I guess I am kinda surprised that no one has picked up on my observation about the classic stereotype of the guitarist who doesn’t understand their gear. There are so many examples - and it is a big reason why “new innovations” in gear get picked up and touted by players, when they really, really suck. Eddie Van Halen is a brilliant, once-in-a-generation guitarist who pulled brilliant sounds out of an erector-set guitar. But so many clueless losers (me included at age 17) took so much of what he did as gospel that guitar design was set back to the stone age. If Eddie played a flat, unfinished neck, left off the tone control and installed a locking whammy bar - wow, we all have to, too. But in pretty much anyone else’s hands, that’s a recipe for shit tone and meaningless dweedly-dweedly wanking…
And beyond! (I’m sorry.)
It took me 11. 
You know, it’s such a fine line between stupid and clever.
What can I say? It’s one of those days when a tight rope looks appealing. 