Year	Number of
1990	4.02
1991	4.08
1992	4.17
1993	4.28
1994	4.4
1995	4.54
1996	4.62
1997	4.68
1998	4.7
1999	4.73
2000	4.78
2001	4.95
2002	5.14
2003	5.31
2004	5.45
2005	5.58
2006	5.82
2007	6.08
2008	6.31
2009	6.58
2010	6.89
2011	7.17
2012	7.49
2013	7.83
2014	8.19
2015	8.57
2016	9.01