Count all those whose youthclub performed at the 14-15 La Liga?
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/ontology/season> <http://dbpedia.org/resource/2014–15_La_Liga> . ?uri <http://dbpedia.org/property/youthclubs> ?x  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Athlete>}
2014–15_La_Liga