Year	Number of
1973	11120
1974	11540
1975	11920
1976	12230
1977	12450
1978	12640
1979	12800
1980	12970
1981	13090
1982	13250
1983	13480
1984	13740
1985	14060
1986	14320
1987	14610
1988	14910
1989	15230
1990	15530
1991	15770
1992	16050
1993	16400
1994	16840
1995	17320
1996	17660
1997	18010
1998	18260
1999	18530
2000	18910
2001	19330
2002	19820
2003	20380
2004	20910
2005	21350
2006	21800
2007	22160
2008	22500
2009	22730
2010	22960
2011	23090
2012	23220
2013	23410
2014	23610
2015	23790