Year	Number of
1990	7.44
1991	7.76
1992	8.02
1993	8.14
1994	8.23
1995	8.31
1996	8.34
1997	8.17
1998	8.05
1999	8.18
2000	8.35
2001	8.58
2002	8.9
2003	9.34
2004	10.04
2005	10.66
2006	11.34
2007	12.02
2008	12.57
2009	13.19
2010	13.69
2011	14.49
2012	15.41
2013	16.34
2014	17.2
2015	18.29
2016	19.33