Year	Number of
1990	18.33
1991	17.98
1992	18.06
1993	18.5
1994	18.6
1995	18.22
1996	19.03
1997	19.79
1998	20.55
1999	20.99
2000	21.78
2001	20.68
2002	20.37
2003	21.05
2004	21.3
2005	22.24
2006	22.02
2007	21.96
2008	22.17
2009	23.01
2010	23.41
2011	23.45
2012	23.48
2013	24.29
2014	24.85
2015	25.48
2016	25.46