Except Excalibur is a magic sword. To borrow legal terminology, it's a material good; you can only use a single copy of an Excalibur in one place at a time, and its properties are because it's a supernaturally empowered artifact. If you made an identical-looking sword, it still wouldn't be Excalibur, unless it was also empowered by a whatever.
Necronomicon is not a material good. You need to have a copy to use it, but it doesn't matter which copy. A book is not anything special in itself, it's not a magical scroll, what matters is the knowledge represented as writing on the paper, but if you wanted to be a bit postmodern, a freaking audiobook or a pdf would work just as well.
There are no books titled the same but completely different in the stories; all Necronomicons are the same book, written by Alhazred, but the various copies that pop up can be translations of various quality or derivative works like the Dee abridged version - it's actually similar to the Bible IRL, your best bet if you know the language is to read the original, because there are various mistranslations or nuances lost in the process.
I'm not sure where you take your number from, and am genuinely curious. For reference, Derleth's work don't count because Derleth was a fucking hack whose only good contribution to the mythos ever was preserving the original Lovecraft stories in the public consciousness.
Pseudoedit: ninja'd. Also it's Alhazred.