From the Wikipedia page 
The Hamburg Metropolitan Region (German: Metropolregion Hamburg) is a metropolitan area centred around the city of Hamburg in northern Germany, consisting of eight districts (Landkreise) in the federal state of Lower Saxony, six districts (Kreise) in the state of Schleswig-Holstein and two districts in the state of Mecklenburg-Vorpommern along with the city-state of Hamburg itself. It covers an area of roughly 26,000 square kilometres (10,000 sq mi) and is home to more than 5.1 million inhabitants.
On 1 January 2006 the office of the Hamburg Metropolitan Region opened, as agreed in a state treaty of cooperation (Staatsvertrag über Zusammenarbeit) between Hamburg, Lower Saxony and Schleswig-Holstein.
As of 2005, the Hamburg Metropolitan Region was made up of the city of Hamburg along with numerous rural districts in Lower Saxony and Schleswig-Holstein, altogether comprising 800 cities, towns and municipalities with an overall land area of 19,802 square kilometres (7,646 sq mi). Since then it has expanded to cover the districts of Ludwigslust-Parchim (partially from 2012, entirely from 2017) and Nordwestmecklenburg (from 2012) in the state of Mecklenburg-Vorpommern.