Year	Number of
1990	14.84
1991	15.11
1992	15.42
1993	15.66
1994	15.97
1995	16.35
1996	16.63
1997	16.98
1998	17.31
1999	17.67
2000	18.06
2001	18.02
2002	18
2003	18.18
2004	18.34
2005	18.47
2006	18.38
2007	18.28
2008	18.22
2009	18.2
2010	18.23
2011	18.47
2012	18.76
2013	19.06
2014	19.36
2015	19.65
2016	20.01