Year	Number of
1990	4.21
1991	4.4
1992	4.58
1993	4.77
1994	4.96
1995	5.15
1996	5.55
1997	5.97
1998	6.42
1999	6.88
2000	7.37
2001	7.94
2002	8.58
2003	9.23
2004	9.92
2005	10.66
2006	11.12
2007	11.58
2008	12.07
2009	12.62
2010	13.23
2011	13.54
2012	13.92
2013	14.34
2014	14.78
2015	15.23
2016	15.69