How many {start time} are for {Iron Age} ?
When did the Iron Age begin?
When did the Iron Age start?
SELECT ( COUNT ( ?u ) AS ?k ) { resource:Iron_Age property:start_time ?u }
SELECT (COUNT(?obj) AS ?objs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q11764> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P580> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
} 

SELECT (COUNT(?obj) AS ?value ) { wd:Q11764 wdt:P580 ?obj }
5