Year	Number of
1990	18.15
1991	17.67
1992	17.76
1993	17.71
1994	17.78
1995	17.7
1996	18.63
1997	16.84
1998	16.73
1999	18.7
2000	17.68
2001	15.71
2002	16.16
2003	16.14
2004	16.11
2005	17.72
2006	18.32
2007	17.66
2008	18.13
2009	18.97
2010	19.53
2011	19.23
2012	19.21
2013	21.39
2014	21.02
2015	21.35
2016	21.4