Year	Number of
1990	2.98
1991	3.2
1992	3.45
1993	3.75
1994	4.1
1995	4.48
1996	4.63
1997	4.79
1998	4.94
1999	5.09
2000	5.25
2001	5.27
2002	5.28
2003	5.32
2004	5.39
2005	5.53
2006	5.66
2007	5.85
2008	6.09
2009	6.34
2010	6.56
2011	6.99
2012	7.37
2013	7.69
2014	8
2015	8.32
2016	8.83