How many {cast member} are for {M} ?
M has how many cast members?
How many cast members are there in M?
SELECT ( COUNT ( ?l ) AS ?y ) { resource:M property:cast_member ?l }
SELECT (COUNT(?obj) AS ?objs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q127021> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P161> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
} 

SELECT (COUNT(?obj) AS ?value ) { wd:Q127021 wdt:P161 ?obj }
48