Count the units garrisoned at Arlington County, Virginia.
SELECT DISTINCT COUNT(?uri) WHERE {?uri <http://dbpedia.org/ontology/garrison> <http://dbpedia.org/resource/Arlington_County,_Virginia>  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/MilitaryUnit>}
Arlington_County,_Virginia