Year	Number of
1990	3.07
1991	3.19
1992	3.3
1993	3.39
1994	3.46
1995	3.55
1996	3.59
1997	3.59
1998	3.6
1999	3.66
2000	3.73
2001	3.91
2002	4.11
2003	4.34
2004	4.63
2005	4.9
2006	5.15
2007	5.41
2008	5.68
2009	5.99
2010	6.27
2011	6.66
2012	7.14
2013	7.67
2014	8.29
2015	8.71
2016	9.15