Year	Number of
1990	3.06
1991	3.22
1992	3.38
1993	3.54
1994	3.7
1995	3.85
1996	3.99
1997	4.13
1998	4.26
1999	4.37
2000	4.49
2001	4.62
2002	4.78
2003	4.96
2004	5.18
2005	5.41
2006	5.63
2007	5.87
2008	6.13
2009	6.42
2010	6.73
2011	7.03
2012	7.35
2013	7.73
2014	8.1
2015	8.45
2016	8.78