Year	Number of
1990	18.17
1991	17.82
1992	17.86
1993	18.19
1994	18.2
1995	17.96
1996	18.43
1997	19.13
1998	19.77
1999	20.03
2000	20.28
2001	19.75
2002	19.6
2003	20.39
2004	20.62
2005	21.59
2006	21.47
2007	21.32
2008	21.49
2009	22.3
2010	22.66
2011	22.65
2012	22.69
2013	23.5
2014	24.08
2015	24.74
2016	24.79