> But if are also consider GNU AT&T syntax

Why, though? The official x86 syntax is Intel syntax. In any case, it doesn't matter. Every possible encoding of each mnemonic is given in Intel's manual. This whole business about "what is an instruction anyway" is just sophistry. Anyone using gas would hopefully know how the AT&T syntax is mapping to Intel encodings.

> but we actually cannot count how many instructions there are in x86/amd64.

Again, you can. It's quite simple. Define an instruction how ever you want, and then go and look at the manual. It's a finite known set. Intel isn't hiding this info.