We run with 2 spares per filer. That is for all our filers, from F840's/12 shelves/36gb drives, F760's/6 shelves/36 gb drives and F760/8 shelves/18gb drives. Since spares are per filer, not volume, the main thing we wanted to avoid was volume failure, which would occur if 2 drives in RAID group failed at the same time. That has never happened, although we almost hit that after moving a filer and 2 disks had a hard time spinning up. We also have 2 because the filers are remote from our offices, so the "insurance" or piece-of-mind are worth it to us.