Year	Number
1970	15400000
1971	15500000
1972	15900000
1973	16100000
1974	16400000
1975	16500000
1976	16900000
1977	17200000
1978	17500000
1979	17700000
1980	17800000
1981	17800000
1982	18200000
1983	18500000
1984	18500000
1985	18700000
1986	19300000
1987	19300000
1988	19400000
1989	19200000
1990	19000000
1991	18800000
1992	18700000
1993	19100000
1994	19700000
1995	19700000
1996	19700000
1997	19200000
1998	16500000
1999	16700000
2000	15700000
2001	16000000
2002	16000000
2003	15900000
2004	17400000
2005	17600000
2006	16900000
2007	16900000
2008	16600000
2009	16600000
2010	17000000
2011	16900000
2012	16800000
2013	18700000
2014	18500000
2015	18200000
2016	18200000
2017	18200000
2018	18000000