1 per city early on, drops to maybe 2 every 3 cities later on, then some extra get built for railroading or if I settle a new landmass (especially if its mostly jungle like in my present game).
edit: early on they tend to work alone, later in groups of 3 (which since I play marathon all the times for building stuff divide by).
Move to tile with 2 workers then start building, or send a worker ahead to road the tile to save the 2nd worker the movement?