How many {cover artist} are to/by {M. C. Escher} ?
How many artists duplicated M.C. Escher's art works?
How many artists covered M.C. Escher's works of art?
SELECT ( COUNT ( ?k ) AS ?q ) { ?k property:cover_art_by resource:M._C._Escher }
SELECT (COUNT(?sub) AS ?subs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> ?sub .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P736> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q1470 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P736 wd:Q1470 }
4