Year	Number of
1990	4.81
1991	4.85
1992	4.85
1993	4.83
1994	4.81
1995	4.81
1996	4.9
1997	5.07
1998	5.25
1999	5.39
2000	5.57
2001	5.72
2002	5.9
2003	6.1
2004	6.33
2005	6.6
2006	6.78
2007	7
2008	7.23
2009	7.47
2010	7.74
2011	8.08
2012	8.44
2013	8.82
2014	9.21
2015	9.62
2016	10.01