How many {incarnation of} are to/by {Vishnu} ?
How many manifestations of Vishnu are there?
How many {incarnation of} {Vishnu} are there?
SELECT ( COUNT ( ?z ) AS ?v ) { ?z property:incarnation_of resource:Vishnu }
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/P3701> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q11380 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P3701 wd:Q11380 }
13