How many {proved by} are to/by {Alexander Grothendieck} ?
Alexander Grothendieck proved how many things?
How much has been proved by Alexander Grothendieck?
SELECT ( COUNT ( ?b ) AS ?n ) { ?b property:proved_by resource:Alexander_Grothendieck }
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/P1318> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q77141 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P1318 wd:Q77141 }
4