Year	Number of
1990	3.74
1991	3.86
1992	4
1993	4.15
1994	4.33
1995	4.53
1996	4.54
1997	4.54
1998	4.58
1999	4.66
2000	4.76
2001	4.88
2002	5
2003	5.12
2004	5.25
2005	5.39
2006	5.64
2007	5.91
2008	6.19
2009	6.5
2010	6.82
2011	7.09
2012	7.38
2013	7.68
2014	8
2015	8.34
2016	9