If you are trying to tax robotic labour by counting robots then you run into some fairly difficult problems. What is a robotic unit? A human unit is easy to identify and measure, but a robot as you point out can be anything.
If you tax per physical unit then instead of building a factory with a hundred independent robots in it, the company will build a fully integrated factory and call it one single robot.
So what else can you do? Tax per kg of robot perhaps, although this would seem to heavily skew the tax burden over into heavy lift tasks and again you'd get out of it by removing the brain part of your 10 ton excavator and putting it into a separate 5kg "pilot bot".
You could tax by algorithmic complexity maybe, however you might measure that. 1 robotic tax unit per human-brain-equivalent. Or tax per robotic manipulator, but do all useful robots have easily countable manipulators?
It would seem very much simpler to me to increase existing taxes on production and compensate employers of humans by reducing the employer tax burden, and leave robot inventors with the freedom to make their robots as production efficient as possible instead of having to tax tailor them for no good reason.