You do not have to fill it out. It is an optional parameter of the Building object. I thought it was used in the model_create_prm_baseline_building method from openstudio-standards to create a 90.1 PRM baseline, but it is not (it instead uses some model_num_stories_spannedmethod and never sets the standardsNumberOfStories parameter.) So I guess it can be some metadata if you want to create measures that read those parameters.

TLDR;

Do not worry about it. It adds additional information to your model, but it is not directly translated into your EnergyPlus simulation.