How many {flower color} are to/by {magenta} ?
How many types of flowers are magenta?
How many flowers' color is magenta?
SELECT ( COUNT ( ?h ) AS ?t ) { ?h property:flower_color resource:Magenta }
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/P2827> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q3276756 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P2827 wd:Q3276756 }
15