Year	Number of
1990	7.44
1991	7.77
1992	8.09
1993	8.33
1994	8.63
1995	8.93
1996	9.14
1997	9.43
1998	9.74
1999	10.13
2000	10.73
2001	11.1
2002	11.63
2003	11.81
2004	12.14
2005	12.74
2006	13.27
2007	13.7
2008	14.03
2009	14.53
2010	14.93
2011	15.42
2012	15.92
2013	16.44
2014	17.04
2015	17.69
2016	18.34