Year	Number
1961	130
1962	150
1963	180
1964	200
1965	250
1966	300
1967	350
1968	400
1969	450
1970	500
1971	550
1972	600
1973	650
1974	768
1975	1058
1976	1316
1977	1636
1978	1900
1979	2100
1980	2200
1981	2250
1982	2300
1983	2350
1984	2400
1985	2430
1986	2450
1987	2480
1988	2500
1989	2550
1990	2600
1991	2650
1992	2700
1993	2700
1994	2700
1995	2700
1996	2700
1997	2700