This is a chest-bumping deal between the later users of the
cell library (who want maximum routing levels and minimum
blockages in the lower levels) and the folks who have to make
all 500 gate primitives laid out at maximum placement density.
The minimum is obviously 1 metal layer, easier if you allow poly
routes. More common is 2 layers (if you are in a technology
with 4 or more).
If you take ownership of the cell library then of course you
can do as you please. But that means becoming responsible
for recharacterizing and requalifying the cell library, which
by your question I think is not something you're prepared for.