Year	Number
1971	100
1972	200
1973	400
1974	300
1975	400
1976	500
1977	600
1978	600
1979	600
1980	300
1981	400
1982	300
1983	300
1984	300
1985	500
1986	1400
1987	1100
1988	700
1989	700
1990	700
1991	700