How many Senators have tenures of over 50 years?
{'directed': True, 'multigraph': False, 'graph': {}, 'nodes': [{'type': 'INTERNAL', 'name_': 'Q30', 'id': 0, 'label': 'United States of America'}, {'type': 'ANSWER_CANDIDATE_ENTITY', 'name_': 'Q200', 'id': 1, 'label': '2'}, {'type': 'QUESTIONS_ENTITY', 'name_': 'Q4416090', 'id': 2, 'label': 'United States senator'}, {'type': 'INTERNAL', 'name_': 'Q456794', 'id': 3, 'label': 'Rufus King'}, {'type': 'INTERNAL', 'name_': 'Q23806', 'id': 4, 'label': '10'}, {'type': 'INTERNAL', 'name_': 'Q3644601', 'id': 5, 'label': 'Brie Garcia'}], 'links': [{'name_': 'P17', 'source': 0, 'target': 0, 'label': 'country'}, {'name_': 'P17', 'source': 2, 'target': 0, 'label': 'country'}, {'name_': 'P27', 'source': 3, 'target': 0, 'label': 'country of citizenship'}, {'name_': 'P39', 'source': 3, 'target': 2, 'label': 'position held'}, {'name_': 'P166', 'source': 3, 'target': 4, 'label': 'award received'}, {'name_': 'P5236', 'source': 4, 'target': 1, 'label': 'prime factor'}, {'name_': 'P27', 'source': 5, 'target': 0, 'label': 'country of citizenship'}, {'name_': 'P734', 'source': 5, 'target': 1, 'label': 'family name'}]}