You mean the Corna? (IDK what Dio cals it..Maluk or something?)
Not many. I would guess under 10%
Plus it has taken on many meanings over the years. Originally it was literally "The Devil's Horns", and was used as both a sign to "Ward off Ill Omens" and also to mark Infedelity. Some attribute it to the Cretan legend of the Minotaur even.
Anton LeVay also used it as literally the Satanic Salute, and it was almost a worshipful sign around the Temple of Set.
Some MEtal bands have taken it to literally represent the Devil's Horns, and it is debated if KISS or Dio (or Blackie Lawless or any other MEtalhead from the 70s or Before. I hear of other bands doing it.) really started the popular use, though Gene Simmons has done it since the 70s, and his "explenation" is that he was using the Sign for "LOVE" but had to hold onto his pick, so he could not extend his thumb.
It has, in more recent years become more of a generic catchall sign used by devotess of Heavy Metal, to basically say "Rock On". Rather akin to how the Peace Sign has evolved over the years.
So, many people do NOT know the signifigance or history of that particular sign (though the problem is not so much with the gesture, but what it represents), and even many who DO know don't actually believe in the Devil and so consider it harmless.
It has been said that the greatest deception the Devil ever managed was convincing the world he did not exist. It is the same reason kids these days see nothing worrisome with hyper gory or Satanic lyrics. The don't believe in it, so they feel it is just harmless fun. Amazes me how many "Atheists" i know who don't believe in that stuff, but then are superstitious as all get out, or consult Oija boards and the like? I mean really...if one exists.. why wouldn't the other?
As for the :"Sign Language" meaning, it has none. LOVE is done with the thumb extended. (I have taken over 3 years of American Sign Language, and have acted as a Minister in the Deaf Community). Well..not offically true. It is used in ASL Slang to mean "BullCrap" in it's cruder form.
BUT, it IS used by fans of the Texas Longhorns College Football teams to mean "Hook Em Horns".
I hope this clarifies things, but if anyone needs anything further xplained, lemme know.
***WOOT!!! UT OH!!! The TD troll is loose again!!!! Oh NOZE! What do we do?!?!?!?!?!?!
though if I ever get to do a documentary on Metal, it would be called "A Fistful of Metal"... which is what I kinda call the sign.