I define "Grail Knife" by specific type or category, so that it is possible for there to be more than one. Once I've acquired a specific Grail, I have no more need for any further examples in that category.
I have two Grail knives in my possession:
The first is a pristine early Gerber Mark 2 with cats-tongue handle, canted and wasp-waisted blade accompanied by a serial-number-appropriate sheath and sharpening stone. I've had it for a couple of decades now, so there are no other Gerber Mark 2's in my collection.
The second is a recent acquisition -- a WWII USMC-marked Mark II ("Ka Bar") by Robeson Shuredge, which is by far the rarest of the USMC-marked Mark II knives. I still have a few other USMC-marked Mark II's but am gradually divesting myself of them. My personal Mark II's are user blades, so they are either USN-marked or post-WWII knives.