By code in most jurisdictions, a deck or porch stairway must be at least 36 inches wide.
Regardless of the width, the stringer span is limited (see illustration above). The design values for visually graded southern pine (see Southern Pine Design Values) dictate the maximum span for treated wood stair stringers.
Stringer attachment. Best practice is to attach the stringer with full bearing against the deck header, and use a metal connector.
Note in photo above that only rim joists made with 2x12s (or bigger) provide enough attachment surface for use with metal stringer mounting brackets.
At the bottom of the stair, stringers should rest on a footing (concrete pads or piers post footing) designed to resist a 40 psf live load and a 10 psf dead load.
For more on this topic, see Building and Installing Deck Stairs.