Hi KingCrimson, first of all thank you, but you really need to thank devonjones for designing these tiles. I just made them compatible with openforge 2.0. There is no "complete set" on this one. I'd recommend you to take a piece of checkered paper and just make a plan for yourself on how many tiles you need for different displays. I personally go for around 50 -75 tiles per set, this is where I feel comfortable building different dungeons. (But I, as a DM, only use terrain for encounters only- I never display full dungeons. I prefer to rely on the narrative at most parts) But it really depends. For "starter set" to display a sewer segment, that just goes around one corner (which might be enough to display a smaller encounter), I'd say you need around 14 wall tiles (also piped if you will), 4 straight sluices, 1 curved, 2 ends, 1 Connector and 5 corners. I will also add sewer entrances that will work as normal walls as well. If you want to have a little more options I'd say you double or triple that, and add some Sluice Tee's or Crosses and some extra Connector Tiles in the Mix. Also add some extra Cut Stone Floors if you want to expand the width. (You'll need one per tile on the outside margin). This really adds up to a lot.
But again, take that piece of paper, and plan it out for yourself. Nobody can really tell you how much you'd need, for your own purposes.