Year	Number of
1990	4.11
1991	4.23
1992	4.35
1993	4.48
1994	4.62
1995	4.77
1996	5
1997	5.25
1998	5.52
1999	5.79
2000	6.08
2001	6.28
2002	6.47
2003	6.66
2004	6.86
2005	7.08
2006	7.3
2007	7.53
2008	7.8
2009	8.1
2010	8.43
2011	8.56
2012	8.71
2013	8.88
2014	9.05
2015	9.25
2016	9.45