Big landmass maps where you have room to expand, 2 per city til you have 10, then 1 more every 2 cities.
Island maps, 1.5 per city til you have 7 or 8. Then its actually more important to transport the ones you have around rather than having too many sitting around doing busy work sucking up maintenance cash. There arent as many tiles for a worker to improve, and half your worker hammers should be going into workboats.
On more packed maps like Pangaea, you can usually get away with a few less, especially if you plan to rush. Get those first few out ASAP though, you wont regret it. Chop distant forests if you need stuff to do, or pre-road paths to future cities, but its better to have too many workers than too few.