Year	Number of
1990	3.38
1991	3.6
1992	3.83
1993	3.99
1994	4.14
1995	4.23
1996	4.3
1997	4.72
1998	5.02
1999	5.4
2000	5.46
2001	5.64
2002	5.74
2003	5.86
2004	5.96
2005	6.05
2006	6.37
2007	6.76
2008	7.21
2009	7.53
2010	7.82
2011	8.23
2012	8.68
2013	9.36
2014	9.74
2015	9.83
2016	10.07