Year	Number of
1990	2.6
1991	2.69
1992	2.78
1993	2.87
1994	2.96
1995	3.06
1996	3.14
1997	3.23
1998	3.31
1999	3.39
2000	3.48
2001	3.59
2002	3.72
2003	3.87
2004	4.03
2005	4.2
2006	4.38
2007	4.55
2008	4.75
2009	4.96
2010	5.21
2011	5.39
2012	5.6
2013	5.85
2014	6.11
2015	6.37
2016	6.66