Year	Number of
1990	3.88
1991	4.01
1992	4.16
1993	4.24
1994	4.33
1995	4.41
1996	4.44
1997	4.44
1998	4.5
1999	4.55
2000	4.63
2001	4.95
2002	5.3
2003	5.73
2004	6.17
2005	6.59
2006	6.96
2007	7.36
2008	7.8
2009	8.29
2010	8.74
2011	9.29
2012	9.9
2013	10.44
2014	11.14
2015	11.67
2016	12.33