If the question was how many $5 rPi ZERO's do you need to mine btc then, surely, the answer is just one NOT cpu mining is dead. Is it worth it? Beauty lies in the eye of the beholder .... usb stick or rPi ZERO ... both have an equal chance of finding a block but the rPi ZERO has the added advantage of being re-useable.
Anyone who says cpu mining is dead is simply being narrow-minded.
And you're wrong. The question was how many RPI0 to be able to mine a mBTC. If you want to go with the lottery route, there is also better ways to go at it, that both cost less for the hardware and less electricity to run.
Its not a narrow minded thing, its just a fact; mining with CPU is a phase that is over.
You can argue that in our times, it is still possible to cross the sea with a galleon by hiring many people and only using arm power over the course of many weeks.
Sure okay its not impossible... but you could just use a ship with an engine or you know, a plane. Thats why its not done anymore.