How many workers do I need for a given number of cities in the early stages of Civ 4, say when I have around 5 cities and the biggest is around size 5?
Consider the question differently: Are all of my citizens working fully improved squares?
A quick rule of thumb from my experience is to have one work per city, always working for that city (unless something like urgent need for a resource arises). Then watch the city and consider: is the worker keeping up with city growth? If he's not, build another in that city, and it'll help slow the growth and gain you a worker.
You will also sooner or later want 2 or 3 workers connecting cities with roads, building forts, etc... things not linked to one city. You can often gain these when cities stop growing, cause that city's worker can actually finish enough squares! Then he can move on to other work. If not, you can always just build a worker for that specifically.
All of this is if you want the BEST cities. Keeping every citizen working a fully improved square is the ultimate goal, but if you spend your resources on that, you're not doing something else. There are many ways to play Civ4, and having the best cities is only one of them. Prioritize accordingly.
My estimate from the info you gave: 7 or 8 workers is the most you'll need. 1 per city, and 2-3 on national road detail/helping with a fast growing city. You probably don't need the 8th if one of your other cities has stopped growing and the worker finished.
Finally, remember that one of the best ways to build workers is to take them from your enemies!