Year	Number of
1990	15.38
1991	15.7
1992	16.09
1993	16.46
1994	16.91
1995	17.38
1996	17.87
1997	18.36
1998	18.77
1999	19.06
2000	19.28
2001	19.07
2002	19.12
2003	19.32
2004	19.4
2005	19.51
2006	19.42
2007	19.07
2008	18.81
2009	18.67
2010	19.11
2011	19.58
2012	20.01
2013	20.31
2014	20.52
2015	20.7
2016	20.94