Year	Number
1961	4
1962	4
1963	4
1964	4
1965	4
1966	4
1967	4
1968	4
1969	4
1970	4
1971	4
1972	4
1973	4
1974	4
1975	5
1976	5
1977	5
1978	5
1979	5
1980	6
1981	6
1982	6
1983	6
1984	6
1985	7
1986	7
1987	7
1988	7
1989	8
1990	8