The load supporting requirements for stair treads are more severe  than for the rest of the deck because the force exerted by a person walking down stairs is more intense blow to the surface than a standard stride across a level surface.  Because of this you should never space your stair stringers more than 16” on center.  This means you will need to install a minimum of four stringers on any stairway wider than 36”.  Always check the maximum span for your decking material as it applies to stair treads. 

Some composite materials and lower grade 5/4 wood decking may require you to reduce the stringer spacing to 12” on center.  It is the best policy to overbuild your stairs because of the increased wear and tear they often receive. Pay particular attention to the stringer placement if you are attaching stairs around a corner as in the case of a cascading staircase.