Year	Number of
1990	6.18
1991	6.31
1992	6.45
1993	6.58
1994	6.74
1995	7.28
1996	7.23
1997	7.98
1998	8.27
1999	8.31
2000	8.73
2001	9.09
2002	9.46
2003	9.9
2004	10.02
2005	10.76
2006	11.23
2007	11.7
2008	12.3
2009	12.97
2010	13.95
2011	14.68
2012	15.54
2013	16.39
2014	16.77
2015	17.25
2016	17.94