How many {participant of} are to/by {World War II} ?
Who participated in WWII in total?
How many people were participants of WWII?
SELECT ( COUNT ( ?r ) AS ?y ) { ?r property:participant_in resource:World_War_II }
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/P1344> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q362 .
} 

SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P1344 wd:Q362 }
44