I think the fewest amount of schools a client of mine has applied to was three (Harvard, Yale, NYU) because said client had to be in that area for personal reasons, and if you are only going to go to one of those schools you have no need for scholarship leverage (Yale/Harvard no giveth). For the record client got into all three, but this is a rare thing to do and I would never bank on it. We just sort of got lucky.
In general, just about everyone should do the classic strategy of "reaches, mid-ground, and safeties"...particularly in this world of scholarship negotiation.
There are some cases, though, where applying to t-6 makes sense. In fact, where it makes sense as a specific strategy. But It is not something that I would recommend without knowing the person super well.