How many {operating system} are to/by {iOS} ?
What number of iOS operating systems are there?
How many devices operating systems are iOS?
SELECT ( COUNT ( ?x ) AS ?d ) { ?x property:operating_system resource:iOS }
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/P306> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q48493 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P306 wd:Q48493 }
749