How many artists have collaborated with famous guitar players?
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/property/notableInstruments> <http://dbpedia.org/resource/Guitar> . ?x <http://dbpedia.org/ontology/associatedMusicalArtist> ?uri  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Artist>}
Guitar