Year	Number of
1990	4.32
1991	4.51
1992	4.69
1993	4.86
1994	5.05
1995	5.28
1996	5.37
1997	5.5
1998	5.6
1999	5.71
2000	5.81
2001	5.79
2002	5.77
2003	5.78
2004	5.81
2005	5.85
2006	6
2007	6.15
2008	6.37
2009	6.59
2010	6.83
2011	7.12
2012	7.44
2013	7.73
2014	8.05
2015	8.41
2016	8.79