Question: Terry Pratchett has written about how many topics?
SPARQL query:
PREFIX prop: <http://dbpedia.org/property/>
PREFIX onto: <http://dbpedia.org/ontology/>
SELECT DISTINCT COUNT(?uri) WHERE { ?x prop:author <http://dbpedia.org/resource/Terry_Pratchett> . ?x onto:nonFictionSubject ?uri  . }