Complex sentence: Hockenheim is one of the six largest towns in the Rhein-Neckar-Kreis district; since 1999 the number of inhabitants exceeded the 20,000 threshold, thus getting the status of a regional central town ("Große Kreisstadt") in 2001.
Split into simpler sentences: Hockenheim is one of the six largest towns in the Rhein-Neckar-Kreis district. Since 1999 the number of inhabitants exceeded the 20,000 threshold, thus getting the status of a regional central town ("Große Kreisstadt") in 2001.