Hi zjam,
In order to calculate the number of rectifiers and groundbeds required for an underground pipe installations there is much information required before the calculations can be done. This information would include where is the pipe going to be installed (I.E. desert, temperate, coastal etc.) Carrying out a pipeline survey along the pipe route, to include soil resistivity, ground acidity, and bacterial presence as a minimum. What size of pipe to be used. What length of pipe. What type of coating. Are there any existing pipes or services along the pipe route? Will the pipe cross any existing services. Are there any high voltage cable routes adjacent (within 1000m)or crossing the pipe?. Are there any other Direct Currents sources that may interact or interfere with the protection of your pipe?
There are many considerations to be considered when using Cathodic Protection for pipeline corrosion protection. Care must be taken to ensure not only getting the right number of rectifier and groundbeds, but to ensure other issues are mitigated such as AC and DC interference. I agree with Ed, retain the services of a competant Cathodic Protection Engineer to guide you through the process.
Hope this is of some help.