You only need to get into one place, so 8 should be more than fine if you picked them well.
I applied to all programs where I found a very good overlap with faculty research interests. It ended up being 13 departments. But if it had been eight, I would have applied to eight. In a way, it depends on how specialized your research interests are.
I think it would have been a waste of money if you had applied to programs even though you knew you would not be a very excellent fit there. More likely than not, those programs would have rejected you anyway.