Year	Number of
1990	5.32
1991	5.38
1992	5.45
1993	5.54
1994	5.65
1995	5.77
1996	6.04
1997	6.34
1998	6.66
1999	7.01
2000	7.38
2001	7.58
2002	7.8
2003	8.06
2004	8.34
2005	8.65
2006	9.12
2007	9.63
2008	10.18
2009	10.8
2010	11.51
2011	12
2012	12.54
2013	13.1
2014	13.62
2015	14.09
2016	14.64