There isn't necessarily a distance limit. Basically you are preventing galvanic corrosion via use of zinc which has a higher electric potential than the metals you are trying to protect. The corrosion occurs because you have two dissimilar metals (different potentials) in the precense of an electrolye (similar to a galvanic cell or battery). The electrolytic solution will serve as a means of transporting ions from the more reactive metal to the less reactive metal. A voltage difference is set up between the metals and the potential across the solution will carry the ions as discussed. What sets the "distance" which you've discussed is the mobility of the ions in the electrolyte solution. If the electrolyte solution has a high resistivity the ions will not readily be transported. In this case, you will see little to no benefit from the zinc. For high resistivity electrolyte solutions, typically a voltage (a DC current is normally used) is impressed on the system to overcome the high resistivity of the electrolytic solution. When used in conjection with zinc anodes, this ensures the zinc will corrode vice the metals you are trying to protect. You typically only see this on very large structures, such as ocean freightliners or pipelines.
So yes, one zinc anode will protect a large area, so long as the resistivity of the electrolyte solution isn't terribly high. If the resitivity is high, you will need to use more anodes, more tightly spaced. The zincs are evenly spaced to ensure the resistance from each zinc to the surrounding area is roughly equivalent, otherwise, you will see one zinc corrode faster than the others. If the structure is very large or your solution is of low conductivity, you may need to use impressed current for protection. It is difficult to say how many anodes you need without knowing the conductivity of the solution, the surface area you are concerned with, and the metals you are attempting to protect.
