Year	Number
1970	2200
1971	2800
1972	2900
1973	3000
1974	3100
1975	3100
1976	6800
1977	7000
1978	7100
1979	7100
1980	9200
1981	9300
1982	7900
1983	9500
1984	9900
1985	9900
1986	15300
1987	13700
1988	14600
1989	13800
1990	14100
1991	11300
1992	13900
1993	13100
1994	12400
1995	4900
1996	11800
1997	7000
1998	7300
1999	7800
2000	14608
2001	14973
2002	15495
2003	14706
2004	16746
2005	17306
2006	18843
2007	14403
2008	14462
2009	14652
2010	20600
2011	20176.7285260675
2012	16807.4072955955
2013	16932
2014	16056
2015	5126
2016	4538
2017	1669
2018	2342
2020	560
2021	560