Year	Number of
1990	13.58
1991	13.92
1992	14.37
1993	14.83
1994	15.33
1995	15.88
1996	16.21
1997	16.58
1998	16.98
1999	17.41
2000	17.89
2001	17.86
2002	17.86
2003	17.88
2004	17.92
2005	17.98
2006	18.08
2007	18.19
2008	18.31
2009	18.44
2010	18.6
2011	18.64
2012	18.7
2013	18.78
2014	18.86
2015	18.95
2016	19.7