I'm claiming to be "in the know", but this is what we do. 

We follow on 16r planter w/15 shank applicator here. Talking to dealers around the country...it all depends on where you live as to which is "right". To my knowledge, all dealers & farmers around here run 1 less shank than rows they're following. If you follow a 12r planter, you have an 11 shank. I'd say you'd have to be pretty good/lucky (here) to follow a 12r planter w/a 15 shank applicator. But-that's another story.

SO-on our outside (= end row minus 1) shanks, we run knives w/double 3/8" tubes. The rest of the bar is plumbed w/single 3/8" tubes. There is NO knife running in our guess rows...none! The manifolds are plumbed so that there is one extra hose & it goes to each end knife. Seems there's all kinds of setups. Some guys don't run knives behind the wheels because ground can get too hard to penetrate. Some guys run knives on every other row. BUT-if you use a NH3 controller with vapor tubes, placing those vapor tubes can confound the setup slightly.

But anyway, 15 shanks on 16r works fine here.