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.