Year	Number of
1990	7.16
1991	7.82
1992	8.43
1993	8.9
1994	9.19
1995	9.29
1996	9.44
1997	9.61
1998	9.78
1999	9.84
2000	9.95
2001	10.13
2002	10.38
2003	11.07
2004	12.07
2005	12.79
2006	13.43
2007	14.18
2008	14.8
2009	15.4
2010	15.93
2011	16.4
2012	16.86
2013	17.3
2014	17.74
2015	18.18
2016	18.19