Year	Number of
1990	4.74
1991	4.93
1992	5.14
1993	5.14
1994	5.21
1995	5.24
1996	5.25
1997	5.29
1998	5.27
1999	5.47
2000	5.61
2001	5.82
2002	5.96
2003	6.12
2004	6.25
2005	6.17
2006	6.45
2007	6.7
2008	6.91
2009	7.03
2010	7.22
2011	7.36
2012	7.7
2013	8.09
2014	8.68
2015	8.67
2016	8.81