Year	Number of
1990	3.39
1991	3.67
1992	4
1993	4.37
1994	4.79
1995	5.24
1996	5.65
1997	5.94
1998	6.18
1999	6.33
2000	6.5
2001	6.56
2002	6.69
2003	6.82
2004	6.89
2005	6.98
2006	7.13
2007	7.32
2008	7.6
2009	7.94
2010	8.26
2011	8.43
2012	8.55
2013	8.79
2014	9.13
2015	9.45
2016	9.81