Year	Number of
1990	3.2
1991	3.33
1992	3.46
1993	3.6
1994	3.76
1995	3.93
1996	4.11
1997	4.31
1998	4.52
1999	4.75
2000	4.99
2001	5.21
2002	5.44
2003	5.68
2004	5.94
2005	6.23
2006	6.43
2007	6.65
2008	6.87
2009	7.09
2010	7.3
2011	7.47
2012	7.65
2013	7.83
2014	8.03
2015	8.28
2016	8.55