The National History Museum of Montevideo is located in the historical residence of General Fructuoso Rivera. It exhibits artifacts related to the history of Uruguay. In a process begun in 1998, the National Museum of Natural History (1837) and the National Museum of Anthropology (1981), merged in 2001, becoming the National Museum of Natural History and Anthropology. In July 2009, the two institutions again became independent. The Historical Museum has annexed eight historical houses in the city, five of which are located in the Ciudad Vieja. One of them, on the same block with the main building, is the historic residence of Antonio Montero, which houses the Museo Romantico.
How many houses has the Historical Museum annexed?
eight