How many {doctoral advisor} are to/by {Thomas Hunt Morgan} ?
How many people was Thomas Hunt Morgan a doctoral advisor for?
Thomas Hunt Morgan is a doctoral advisor to how many?
SELECT ( COUNT ( ?r ) AS ?f ) { ?r property:doctoral_advisor resource:Thomas_Hunt_Morgan }
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/P184> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q177681 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P184 wd:Q177681 }
6