Year	Number of
1990	3.72
1991	3.83
1992	3.89
1993	4.03
1994	4.1
1995	4.27
1996	4.39
1997	4.5
1998	4.65
1999	4.89
2000	5.05
2001	5.19
2002	5.37
2003	5.57
2004	5.81
2005	6.02
2006	6.31
2007	6.59
2008	6.87
2009	7.16
2010	7.52
2011	7.81
2012	8.12
2013	8.44
2014	8.76
2015	9.05
2016	9.35