Year	Number of
1990	12.5
1991	13.16
1992	13.81
1993	14.48
1994	15.27
1995	15.97
1996	16.85
1997	16.96
1998	17.05
1999	18.02
2000	19.27
2001	19.55
2002	20.15
2003	20.47
2004	20.81
2005	20.29
2006	20.43
2007	20.38
2008	20.17
2009	20.13
2010	20.33
2011	20.66
2012	21.07
2013	20.77
2014	21.21
2015	21.32
2016	21.94