Year	Number
1961	31
1962	31
1963	31
1964	31
1965	33
1966	60
1967	80
1968	100
1969	130
1970	150
1971	170
1972	200
1973	230
1974	250
1975	280
1976	300
1977	350
1978	449
1979	421
1980	572
1981	675
1982	700
1983	600
1984	600
1985	600
1986	500
1987	500
1988	500
1989	500
1990	508
1991	508