Well, I have no exact figures in hand, but I will tell you that in my closed system with anywhere from 15 to 17 adult and juvenile clams, my nitrates are depleted rapidly. I am also constantly battling to keep my phosphates up. This is my biggest problem. I test my water every 3 days or so, and always get an undetectable reading of phosphate.
This is, of course, undesirable for clams. I have a heavy fish load to try and compensate for this, and I sometimes turn off my skimmer.
With a large number of clams you will notice a marked decrease in "measurable" elements and nutrients. With one or two in a large system I doubt they would make a significant, testable difference in water quality.