Year	Number of
1956	15330
1957	16310
1958	17350
1959	18340
1960	19210
1961	19810
1962	20270
1963	20620
1964	20900
1965	21220
1966	21630
1967	22020
1968	22430
1969	22880
1970	23340
1971	23610
1972	23850
1973	23960
1974	24010
1975	23830
1976	23580
1977	23320
1978	23070
1979	23060
1980	23250
1981	23340
1982	23630
1983	23960
1984	24210
1985	24280
1986	24320
1987	24200
1988	23970
1989	23710
1990	23530
1991	23400
1992	23410
1993	23460
1994	23510
1995	23520
1996	23370
1997	23150
1998	22990
1999	22760
2000	22700
2001	22790
2002	22970
2003	23420
2004	23840
2005	24300
2006	24640
2007	24820
2008	24950
2009	25060
2010	25200
2011	25370
2012	25620
2013	25720
2014	25710
2015	25400