Year	Number of
1990	14.23
1991	14.56
1992	14.79
1993	15
1994	15.2
1995	15.59
1996	16.42
1997	16.84
1998	16.91
1999	16.93
2000	17.31
2001	17.52
2002	17.83
2003	18.19
2004	18.78
2005	18.92
2006	18.61
2007	18.68
2008	19.18
2009	19.47
2010	19.1
2011	18.67
2012	18.34
2013	18.24
2014	18.61
2015	19.19
2016	19.9