Lagos is the most populous city in Nigeria, the second fastest growing city in Africa and the sixth most populous in the world.The population of Lagos urban area, according to the Lagos State Government is 17.5 million, a number disputed by the Nigerian Government and judged unreliable by the National Population Commission of Nigeria. Lagos was reported in 2014 to have a metropolitan population of 21 million, making Lagos the largest metropolitan area in Africa.

See more here.

Other helpful links

Connect Nigeria: 5 amazing places to visit in Lagos
Trip Advisor: Fun activities in Lagos