Year	Number of
1990	3.35
1991	3.5
1992	3.71
1993	4
1994	4.36
1995	4.76
1996	5.33
1997	5.84
1998	6.22
1999	6.49
2000	6.73
2001	6.89
2002	7.16
2003	7.44
2004	7.61
2005	7.8
2006	7.92
2007	7.93
2008	7.91
2009	8.11
2010	8.41
2011	8.63
2012	8.92
2013	9.2
2014	9.5
2015	9.6
2016	9.74