Year	Number of
1990	3.96
1991	4.07
1992	4.2
1993	4.34
1994	4.5
1995	4.65
1996	4.8
1997	4.95
1998	5.12
1999	5.31
2000	5.51
2001	5.55
2002	5.62
2003	5.7
2004	5.78
2005	5.88
2006	6.03
2007	6.19
2008	6.36
2009	6.56
2010	6.77
2011	6.99
2012	7.22
2013	7.46
2014	7.71
2015	7.99
2016	8.28