Year	Number of
1990	2.43
1991	2.51
1992	2.57
1993	2.64
1994	2.7
1995	2.77
1996	2.92
1997	3.08
1998	3.24
1999	3.37
2000	3.5
2001	3.6
2002	3.72
2003	3.87
2004	4.04
2005	4.22
2006	4.39
2007	4.59
2008	4.81
2009	5.02
2010	5.25
2011	5.49
2012	5.76
2013	6.04
2014	6.34
2015	6.66
2016	6.97