Any number should be fine if we actually had proper standards. Why are graphics so special that we can't have x86 of graphics? I think all of this is a problem of this klepto capitalism. Also, you need 4 to have competition
What problem do you think having a standard ISA would solve? Nobody distributes native binaries for GPUs, only shaders (either compiled to intermediate representation or no). From my perspective, all that a standard ISA would cause is less implementation flexibility in the hardware since now the hardware has to deal with compatibility instead of letting the vendor-specific compiler included with the driver deal with it.