Year	Number of
1990	6.56
1991	6.83
1992	7.03
1993	7.23
1994	7.43
1995	7.65
1996	7.77
1997	7.87
1998	7.89
1999	8.04
2000	8.12
2001	8.28
2002	8.41
2003	8.56
2004	8.71
2005	8.81
2006	8.87
2007	8.94
2008	8.99
2009	9.08
2010	9.22
2011	9.37
2012	9.5
2013	9.55
2014	9.59
2015	9.62
2016	9.78