Year	Number of
1990	5.86
1991	5.91
1992	6
1993	6.13
1994	6.29
1995	6.49
1996	6.79
1997	7.26
1998	7.71
1999	8.23
2000	8.84
2001	9.27
2002	9.7
2003	10.11
2004	10.59
2005	11.08
2006	11.62
2007	12.19
2008	12.82
2009	13.42
2010	14.07
2011	14.67
2012	15.33
2013	16.01
2014	16.75
2015	17.54
2016	18.33