Digital BW, The Print

I am overwelmed by the number of inksets appearing on the market
for printing Fine Art B&W. The go from 3 densities of grays (like
Epson K3) up to 7 densities or so.
My question to the experts (expecially
Paul Roark for his extensive research in the field) is the following:
which is a reasonable number of gray densities needed for an inkset
to make top notch B&W prints?

Thanks,
  Daniela