Year	Number of
1990	5.06
1991	5.32
1992	5.58
1993	5.85
1994	6.14
1995	6.49
1996	6.58
1997	6.71
1998	6.83
1999	6.95
2000	7.07
2001	6.94
2002	6.81
2003	6.71
2004	6.61
2005	6.51
2006	6.79
2007	7.07
2008	7.42
2009	7.78
2010	8.17
2011	8.42
2012	8.7
2013	8.95
2014	9.23
2015	9.54
2016	9.88