Year	Number of
1990	2.49
1991	2.64
1992	2.81
1993	2.94
1994	3.12
1995	3.34
1996	3.38
1997	3.8
1998	4.03
1999	4.17
2000	4.41
2001	4.5
2002	4.54
2003	4.67
2004	4.74
2005	4.99
2006	5.3
2007	5.55
2008	5.83
2009	6.1
2010	6.4
2011	6.72
2012	7.1
2013	7.46
2014	7.71
2015	8.18
2016	8.63