Year	Number of
1990	2.92
1991	3
1992	3.1
1993	3.2
1994	3.29
1995	3.39
1996	3.47
1997	3.55
1998	3.61
1999	3.69
2000	3.77
2001	3.87
2002	4
2003	4.13
2004	4.28
2005	4.45
2006	4.62
2007	4.79
2008	4.98
2009	5.19
2010	5.43
2011	5.59
2012	5.78
2013	6
2014	6.23
2015	6.46
2016	6.71