4 stringers should be enough for a 6 foot wide stairs. If you are using
2X6 as treads, then that means they will span appox. 20" between each
stringer. The max you can span with 2X6 is I believe 24". And make sure
you use 2X12's for stringers, not 2X 10's like some people I've seen.
I'm not sure what frost piers are. Are those the piers that look like
elephant feet for a floating deck? Also what is at the ground level of
the deck, grass?