Year	Number of
1990	1.9
1991	1.96
1992	1.98
1993	2.03
1994	2.04
1995	1.99
1996	2.19
1997	2.44
1998	2.76
1999	3.1
2000	3.33
2001	3.4
2002	3.3
2003	3.29
2004	3.22
2005	3.26
2006	3.39
2007	3.52
2008	3.77
2009	4.03
2010	4.26
2011	4.41
2012	4.6
2013	4.65
2014	4.87
2015	5.21
2016	5.64