building.standardsNumberOfStories just access a field to store optional metadata. It doesn't actually inspect other model elements. There are other options. You could simply do model.getBuildingStorys.size to count story objects, but that may also count building story objects that are not assigned to any spaces. Some other issues are dealing with zone multiplier and plenums/attics. In the openstudio-standards gem there is a model_effective_num_stories method. This addresses many of these issues, and others such as identifying below vs. above grade stories. I believe it does expect spaces to already be assigned to building story objects. If you are working with a generic model that may not have story objects you can use the AssignSpacesToStories measure on BCL which evaluates the z value of floors in spaces, creates a story object for common values, and assign the spaces to the new story.