Year	Number of
1990	2.19
1991	2.29
1992	2.42
1993	2.54
1994	2.65
1995	2.78
1996	2.91
1997	3.01
1998	3.16
1999	3.32
2000	3.45
2001	3.56
2002	3.61
2003	3.73
2004	3.85
2005	3.97
2006	4.15
2007	4.36
2008	4.51
2009	4.71
2010	4.95
2011	5.33
2012	5.57
2013	5.8
2014	6.03
2015	6.25
2016	6.4