Year	Number of
1990	3.13
1991	3.29
1992	3.4
1993	3.56
1994	3.69
1995	3.9
1996	3.6
1997	3.41
1998	3.27
1999	3.21
2000	3.2
2001	3.12
2002	3.28
2003	3.44
2004	3.55
2005	3.76
2006	3.91
2007	4.22
2008	4.75
2009	5.35
2010	5.82
2011	6.14
2012	6.49
2013	6.69
2014	7.03
2015	7.38
2016	8.04