How many {item operated} are to/by {mortar} ?
How many machines are operated by mortar?
SELECT (COUNT(?sub) AS ?value ) { ?sub wdt:P121 wd:Q7905205 }
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/P121> .
?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> http://wikidata.dbpedia.org/resource/Q7905205 .
} 

Mortar operates how many machines?