How many {lake outflow} are to/by {Yukon River} ?
How many tributaries does the Yukon River have?
How many lakes flow out to the Yukon River?
SELECT ( COUNT ( ?a ) AS ?w ) { ?a property:lake_outflow resource:Yukon_River }
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/P201> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q104437 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P201 wd:Q104437 }
4