Year	Number of
1990	4.52
1991	4.71
1992	4.94
1993	5.13
1994	5.26
1995	5.51
1996	5.38
1997	5.4
1998	5.81
1999	5.83
2000	6.03
2001	5.84
2002	5.99
2003	5.96
2004	6.04
2005	6.12
2006	6.35
2007	6.47
2008	6.74
2009	6.94
2010	7.18
2011	7.51
2012	7.95
2013	8.28
2014	8.8
2015	8.99
2016	9.32