I made a Python package which documents x86(-64) instructions in a ready-to-use way: https://github.com/Maratyszcza/Opcodes (also `opcodes` on PyPI). With this package, its easy to collect ISA stats, e.g.

  import opcodes.x86_64
  isa = opcodes.x86_64.read_instruction_set()
  print(sum(len(instruction.forms) for instruction in isa))
  >>> 6020

As an another example, here is number of instruction forms (e.g. mnemonic name + operand types) over time on Intel CPUs: http://imgur.com/a/AVPcq