How many {has contributing factor} are for {wall} ?
How many people has a contribution factor for the wall?
SELECT (COUNT(?obj) AS ?value ) { wd:Q42948 wdt:P1479 ?obj }
SELECT (COUNT(?obj) AS ?objs ) {
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q42948> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P1479> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj .
} 

For the wall, how many people have a contribution factor?