Year	Number of
1990	5.48
1991	5.52
1992	5.52
1993	5.49
1994	5.46
1995	5.47
1996	5.53
1997	5.65
1998	5.83
1999	6.06
2000	6.33
2001	6.64
2002	7.01
2003	7.45
2004	7.91
2005	8.38
2006	8.91
2007	9.44
2008	10
2009	10.59
2010	11.22
2011	11.89
2012	12.61
2013	13.39
2014	14.22
2015	15.07
2016	15.99