How many politicians are in the US Senate?
SELECT DISTINCT COUNT(?uri) WHERE {?uri <http://dbpedia.org/property/order> <http://dbpedia.org/resource/United_States_Senate>  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/OfficeHolder>}
United_States_Senate