Year	Number
1970	17600
1971	17600
1972	16200
1973	16000
1974	16500
1975	16600
1976	20500
1977	23100
1978	24300
1979	23900
1980	24500
1981	21000
1982	15100
1983	15000
1984	16500
1985	16200
1986	15600
1987	15700
1988	16100
1989	15300
1990	17200
1991	14300
1992	15300
1993	16900
1994	16200
1995	17800
1996	17200
1997	16800
1998	18300
1999	18600
2000	20491
2001	20129
2002	9433
2003	17202
2004	18016
2005	17911
2006	14142
2007	37310
2008	11861
2009	9566
2010	11714
2011	12332.7272727273
2012	13267
2013	9451
2014	10385
2015	8852
2016	7230
2017	7650
2018	10170
2019	10600
2020	4225.333
2021	2038