Year	Number of
1990	6.34
1991	6.45
1992	6.57
1993	6.71
1994	6.85
1995	7
1996	7.17
1997	7.34
1998	7.51
1999	7.68
2000	7.89
2001	8.63
2002	9.41
2003	10.22
2004	11.1
2005	12.08
2006	12.82
2007	13.68
2008	14.66
2009	15.74
2010	16.88
2011	17.63
2012	18.15
2013	18.69
2014	19.24
2015	19.8
2016	20.44