Year	Number of
1990	13.48
1991	13.3
1992	13.01
1993	12.63
1994	12.28
1995	11.91
1996	11.3
1997	11.52
1998	11.69
1999	11.46
2000	12.06
2001	12.19
2002	12.43
2003	12.99
2004	13.28
2005	13.77
2006	14.06
2007	14.46
2008	14.76
2009	15.4
2010	15.8
2011	16.14
2012	16.57
2013	17.07
2014	17.53
2015	17.92
2016	18.34