How many {editor} are to/by {Friedrich Schiller} ?
How many editors are by Friedrich Schiller?
SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P98 wd:Q22670 }
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/P98> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q22670 .
} 

Friedrich Schiller has now may editions?