Year	Number of
1990	8.24
1991	8.51
1992	8.82
1993	9.09
1994	9.46
1995	9.91
1996	10.33
1997	10.69
1998	11.14
1999	11.64
2000	12.14
2001	12.56
2002	13.02
2003	13.68
2004	14.28
2005	14.83
2006	15.41
2007	15.88
2008	16.85
2009	18.51
2010	18.93
2011	19.55
2012	20.11
2013	20.84
2014	21.64
2015	22.4
2016	23.11