Question: how many books are in the missing series by haddix
[{'answer': 'eighth and final book', 'provenance': [{'wikipedia_id': '20797568', 'title': 'The Missing (novel series)', 'start_paragraph_id': 1, 'start_character': 737, 'end_paragraph_id': 1, 'end_character': 758, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '20797568', 'title': 'The Missing (novel series)', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 955, 'bleu_score': 0.803158627699152, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17650136122840401663}, 'section': 'Section::::Abstract.'}]}]