Year	Number
1970	18500000
1971	18700000
1972	19300000
1973	20300000
1974	21100000
1975	21800000
1976	22100000
1977	22700000
1978	23000000
1979	23100000
1980	23300000
1981	23200000
1982	22900000
1983	22600000
1984	22400000
1985	22200000
1986	22000000
1987	21700000
1988	21300000
1989	21100000
1990	20900000
1991	20800000
1992	20900000
1993	21500000
1994	21700000
1995	21700000
1996	21100000
1997	21200000
1998	21900000
1999	21800000
2000	22900000
2001	22900000
2002	22800000
2003	22800000
2004	21100000
2005	21100000
2006	21400000
2007	21100000
2008	20900000
2009	20900000
2010	20900000
2011	20800000
2012	20800000
2013	21100000
2014	21100000
2015	21200000
2016	21200000
2017	21200000
2018	21300000