Year	Number
1961	2
1962	2
1963	2
1964	3
1965	3
1966	3
1967	4
1968	4
1969	5
1970	5
1971	6
1972	6
1973	7
1974	7
1975	9
1976	9
1977	11
1978	11
1979	13
1980	13
1981	15
1982	15
1983	16
1984	18
1985	20
1986	22
1987	24
1988	26
1989	27
1990	29
1991	31
1992	33
1993	35
1994	37
1995	39
1996	41
1997	43
1998	45
1999	46
2000	48
2001	50
2002	52
2003	54
2004	56