So I'm inclined to say you need something like 0.75 worker per city in the early game, and more like 0.25 per city in the late game. At the beginning, 1-2 workers will get it all done in a quick enough time for 1 to 3 cities before they grow to any size that is working a lot of tiles. Then at some point later if you are expanding to 4-5 cities, this is the crucial time when you really need a healthy workforce, generally about 4-5 workers total. But the requirement for a higher ratio of wokers later on drops significantly