Year	Number
1970	15600
1971	17900
1972	19400
1973	17400
1974	15200
1975	15800
1976	17100
1977	13300
1978	17100
1979	17500
1980	17200
1981	15900
1982	18000
1983	20300
1984	17100
1985	16000
1986	18700
1987	19400
1988	17600
1989	18900
1990	19400
1991	12000
1992	9900
1993	8600
1994	12500
1995	12800
1996	14400
1997	16800
1998	19700
1999	15700
2000	12264
2001	11728
2002	9773
2003	8998
2004	10937
2005	11484
2006	11533
2007	11874
2008	11686
2009	10653
2010	21235.4502809087
2011	18302.8583708965
2012	15869.7722047845
2013	23292
2014	15468
2015	17592
2016	20058
2017	13144
2018	8830
2019	7146
2020	2123
2021	756