How many tablets of sodium citrate do I need to take to get the equivalent of one tablet of calcium carbonate?

It depends on the product you are currently taking and the one you plan to take. Most calcium carbonate products available in pharmacies list the amount of elemental calcium, not the amount of calcium salt. For example, m-cal 500 mg, calcite 500 mg, euro-cal 500 mg, etc, all contain 500 mg of elemental calcium. If you want to switch to citrate, it is easy to convert to ci-cal, since it also indicates the elemental calcium content. Therefore, two tablets of ci-cal 250 mg are equivalent to one tablet of calcium carbonate 500 mg.