Year	Number of
1990	18.4
1991	18.58
1992	18.6
1993	17.84
1994	18.02
1995	17.03
1996	17.04
1997	16.74
1998	16.42
1999	16.29
2000	16.54
2001	16.28
2002	16.29
2003	16.22
2004	16.2
2005	15.44
2006	15.29
2007	14.57
2008	14.68
2009	14.67
2010	14.79
2011	15
2012	15.11
2013	15.64
2014	16.63
2015	17.61
2016	18.49