Year	Number of
1990	2.28
1991	2.41
1992	2.54
1993	2.68
1994	2.82
1995	2.97
1996	3.13
1997	3.32
1998	3.53
1999	3.76
2000	4.02
2001	4.28
2002	4.58
2003	4.93
2004	5.31
2005	5.72
2006	5.99
2007	6.26
2008	6.55
2009	6.85
2010	7.18
2011	7.46
2012	7.77
2013	8.09
2014	8.42
2015	8.76
2016	9.39