Year	Number of
1990	2.61
1991	2.78
1992	2.97
1993	3.11
1994	3.29
1995	3.48
1996	3.51
1997	3.96
1998	4.22
1999	4.35
2000	4.57
2001	4.71
2002	4.77
2003	4.92
2004	4.99
2005	5.23
2006	5.56
2007	5.84
2008	6.15
2009	6.41
2010	6.71
2011	7.07
2012	7.52
2013	7.93
2014	8.18
2015	8.68
2016	9.16