Year	Number
1961	1300
1962	1400
1963	1500
1964	1600
1965	1700
1966	1800
1967	1900
1968	2000
1969	2100
1970	2200
1971	2300
1972	2400
1973	2600
1974	2800
1975	3000
1976	3200
1977	3400
1978	3600
1979	3800
1980	4000
1981	4200
1982	4400
1983	4500
1984	4600
1985	4750
1986	4900
1987	5000
1988	5100
1989	5100
1990	5200
1991	5300
1992	5350
1993	5350
1994	5350
1995	5500
1996	5600
1997	5700
1998	5700
1999	5700
2000	6000