In how many different stadiums are the seasons of the 2015 NA Soccer league held?
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/ontology/league> <http://dbpedia.org/resource/2015_North_American_Soccer_League_season> . ?x <http://dbpedia.org/ontology/ground> ?uri  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Stadium>}
2015_North_American_Soccer_League_season