Year	Number of
1990	2.58
1991	2.7
1992	2.83
1993	2.92
1994	3.01
1995	3.24
1996	3.37
1997	3.54
1998	3.86
1999	3.95
2000	3.95
2001	4.06
2002	3.96
2003	4.02
2004	4.09
2005	4.28
2006	4.51
2007	4.82
2008	5.14
2009	5.29
2010	5.55
2011	5.75
2012	6.01
2013	6.25
2014	6.52
2015	6.82
2016	7.06