Year	Number
1970	11300000
1971	12300000
1972	12500000
1973	12700000
1974	12900000
1975	13000000
1976	13600000
1977	13900000
1978	14000000
1979	14400000
1980	14800000
1981	15100000
1982	15400000
1983	15400000
1984	15400000
1985	15500000
1986	15900000
1987	16000000
1988	15800000
1989	15700000
1990	15400000
1991	15400000
1992	15500000
1993	16000000
1994	16800000
1995	16700000
1996	16600000
1997	15500000
1998	13300000
1999	13400000
2000	12400000
2001	12500000
2002	12600000
2003	12500000
2004	13900000
2005	14200000
2006	13400000
2007	13300000
2008	13000000
2009	12700000
2010	12700000
2011	12400000
2012	12100000
2013	13600000
2014	13400000
2015	12900000
2016	12900000
2017	13000000
2018	13000000