How many {practiced by} are for {tennis} ?
How many tennis practitioners are there?
How many people practice tennis?
SELECT ( COUNT ( ?h ) AS ?v ) { resource:tennis property:practiced_by ?h }
SELECT (COUNT(?obj) AS ?objs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q847> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P3095> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
} 

SELECT (COUNT(?obj) AS ?value ) { wd:Q847 wdt:P3095 ?obj }
4