Year	Number of
1990	6.64
1991	6.95
1992	7.24
1993	7.53
1994	7.84
1995	8.16
1996	8.5
1997	8.84
1998	9.19
1999	9.5
2000	9.8
2001	10.08
2002	10.42
2003	10.85
2004	11.33
2005	11.81
2006	12.26
2007	12.83
2008	13.42
2009	13.96
2010	14.49
2011	15
2012	15.55
2013	16.09
2014	16.62
2015	17.17
2016	17.65