Year	Number of
1990	3.79
1991	3.93
1992	4.04
1993	4.21
1994	4.37
1995	4.56
1996	4.79
1997	4.93
1998	5.1
1999	5.22
2000	5.36
2001	5.47
2002	5.62
2003	5.87
2004	6.18
2005	6.55
2006	6.89
2007	7.14
2008	7.42
2009	7.72
2010	8.08
2011	8.51
2012	9.05
2013	9.57
2014	10.15
2015	10.71
2016	11.26