Year	Number of
1990	4.45
1991	4.49
1992	4.58
1993	4.69
1994	4.9
1995	5.06
1996	4.86
1997	4.78
1998	5.03
1999	4.95
2000	5.1
2001	5.03
2002	5.38
2003	5.48
2004	5.54
2005	5.79
2006	5.86
2007	5.86
2008	6.06
2009	6.21
2010	6.57
2011	7.02
2012	7.51
2013	7.85
2014	8.2
2015	8.53
2016	8.98