Question: how many countries are there in west africa
[{'answer': '18', 'provenance': [{'wikipedia_id': '67393', 'title': 'West Africa', 'start_paragraph_id': 1, 'start_character': 507, 'end_paragraph_id': 1, 'end_character': 509, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]