I'm pretty sure Dullstar is thinking about the wrong level, as it is pretty obvious that Wicked 13 requires several builders.
At this point, I'm becoming more and more convinced that the minimum for that level is seven builders. I haven't verified it yet, but I've got evidence from doing each half of the level separately.
So far I've got the left side down to four. The placement and alignment of the chains seems to be prohibiting this from being reduced down to three, as I can't seem to avoid having a builder hit his head on at least one of them.
As for the right side, I'm pretty sure three builders is the minimum. It can be done using the blocker cancelling steel glitch to create a step so the lemmings can get to the exit from the top of the third bridge. I highly doubt two builders is possible for the right side, as that would require the glitch to be repeated multiple times, and I doubt there are enough skills available. Plus there's also the issue of save percentage.