Year	Number of
1990	2.79
1991	2.89
1992	3
1993	3.07
1994	3.15
1995	3.25
1996	3.37
1997	3.53
1998	3.72
1999	3.91
2000	4.1
2001	4.32
2002	4.52
2003	4.74
2004	5
2005	5.18
2006	5.46
2007	5.72
2008	6.01
2009	6.27
2010	6.48
2011	6.78
2012	7.19
2013	7.55
2014	7.94
2015	8.35
2016	8.8