How many people have managed a club which owns the Riverside Stadium?
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/property/ground> <http://dbpedia.org/resource/Riverside_Stadium> . ?uri <http://dbpedia.org/ontology/managerClub> ?x  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Person>}
Riverside_Stadium