Year	Number of
1969	17620
1970	18170
1971	18540
1972	18920
1973	19170
1974	19250
1975	19300
1976	19250
1977	19200
1978	19180
1979	19320
1980	19500
1981	19570
1982	19620
1983	19650
1984	19560
1985	19250
1986	18850
1987	18420
1988	18110
1989	18020
1990	18200
1991	18460
1992	18800
1993	19190
1994	19580
1995	20030
1996	20410
1997	20840
1998	21250
1999	21560
2000	21820
2001	21950
2002	21970
2003	21820
2004	21610
2005	21230
2006	20830
2007	20550
2008	20300
2009	20050
2010	19640
2011	19250
2012	18930
2013	18650
2014	18400
2015	18350