An interesting read but basically boils down to "how many primitive opcodes are there" rather than instructions on the whole. It also doesn't take into account the use of prefixes: for example nop1 and nop2 are technically two instructions (0x90 and 0x66 0x90) but just counting opcodes isn't going to see that.

However if you are interested in assembly this is a well written post and talks about the problem of overloaded opcodes (eg mov, add) and gives different ways of coming up with the answers, which are between 900 and 2500 depending on what you count ...