Year	Number of
1990	8.32
1991	8.45
1992	8.58
1993	8.75
1994	8.95
1995	9.12
1996	9.35
1997	9.67
1998	10.01
1999	10.48
2000	10.77
2001	11.31
2002	11.83
2003	12.44
2004	13.05
2005	13.71
2006	14.44
2007	15.24
2008	16.02
2009	16.76
2010	17.41
2011	17.8
2012	18.21
2013	18.54
2014	18.96
2015	19.51
2016	20.24