Year	Number of
1990	2.73
1991	2.85
1992	2.97
1993	3.11
1994	3.26
1995	3.42
1996	3.54
1997	3.66
1998	3.81
1999	3.96
2000	4.12
2001	4.25
2002	4.38
2003	4.52
2004	4.66
2005	4.82
2006	5.06
2007	5.31
2008	5.59
2009	5.89
2010	6.19
2011	6.53
2012	6.89
2013	7.3
2014	7.74
2015	8.22
2016	8.75