How many games are in the Call of Duty series?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q14915517', 'id': 0, 'label': 'PEGI 18'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q38712', 'id': 1, 'label': '18'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q192156', 'id': 2, 'label': 'Call of Duty'}, {'type': 'INTERNAL', 'name_': 'Q207797', 'id': 3, 'label': 'Call of Duty: World at War'}], 'links': [{'name_': 'P180', 'source': 0, 'target': 1, 'label': 'depicts'}, {'name_': 'P527', 'source': 2, 'target': 3, 'label': 'has part(s)'}, {'name_': 'P908', 'source': 3, 'target': 0, 'label': 'PEGI rating'}, {'name_': 'P179', 'source': 3, 'target': 2, 'label': 'part of the series'}]}