Year	Number
1970	12500000
1971	13500000
1972	13900000
1973	14100000
1974	14300000
1975	14400000
1976	14900000
1977	15300000
1978	15500000
1979	15900000
1980	16200000
1981	16700000
1982	17000000
1983	17000000
1984	17000000
1985	17100000
1986	17500000
1987	17600000
1988	17600000
1989	17600000
1990	17500000
1991	17700000
1992	17900000
1993	18800000
1994	19600000
1995	19900000
1996	19900000
1997	18800000
1998	17200000
1999	17400000
2000	16300000
2001	16500000
2002	16300000
2003	16100000
2004	17300000
2005	17200000
2006	16500000
2007	16600000
2008	16500000
2009	16400000
2010	16800000
2011	16600000
2012	16500000
2013	19800000
2014	19900000
2015	19900000
2016	19900000
2017	20000000
2018	19900000