Year	Number of
1990	2.77
1991	2.94
1992	3.1
1993	3.25
1994	3.41
1995	3.59
1996	3.39
1997	3.23
1998	3.1
1999	3
2000	2.91
2001	2.98
2002	3.06
2003	3.17
2004	3.31
2005	3.5
2006	3.76
2007	4.09
2008	4.49
2009	4.94
2010	5.45
2011	5.82
2012	6.19
2013	6.54
2014	6.85
2015	7.11
2016	7.66