To be honest, a single sample of NSW would probably be sufficient. Yes, there are undoubtedly some differences between the compositions from different oceans, but I would be very surprised indeed if they approach the differences between synthetic salt mixes.
I proposed three different samples of NSW just to characterize the amount of variation and to confirm that it is negligable compared to the mixes.
As to more samples, remember it's $280 per for the analysis...
Why don't we get one locally (New England), one from the west coast (we know several people who could send it to us -- both Tom and I are originally from there), and one from somewhere far off -- maybe from a Reefs.org member who lives in Australia, or the Orient, or something like that?