Year	Number of
1990	4.46
1991	4.48
1992	4.46
1993	4.41
1994	4.37
1995	4.36
1996	4.42
1997	4.53
1998	4.68
1999	4.81
2000	4.96
2001	5.1
2002	5.26
2003	5.45
2004	5.64
2005	5.85
2006	6.02
2007	6.2
2008	6.4
2009	6.62
2010	6.85
2011	7.17
2012	7.51
2013	7.87
2014	8.25
2015	8.64
2016	9.05