Year	Number of
1990	3.38
1991	3.51
1992	3.62
1993	3.75
1994	3.88
1995	4.05
1996	4.07
1997	3.94
1998	3.97
1999	4.26
2000	4.51
2001	4.56
2002	4.73
2003	4.9
2004	5.02
2005	5.08
2006	5.13
2007	5.22
2008	5.44
2009	5.58
2010	5.79
2011	6
2012	6.34
2013	6.7
2014	7.01
2015	7.18
2016	7.37