Year	Number of
1990	5.73
1991	6.17
1992	6.61
1993	7.05
1994	7.44
1995	7.75
1996	7.88
1997	7.87
1998	7.98
1999	8.07
2000	8.25
2001	8.63
2002	9.24
2003	9.97
2004	10.6
2005	10.96
2006	11.85
2007	12.65
2008	13.68
2009	14.95
2010	16.43
2011	17.25
2012	18.16
2013	18.91
2014	19.85
2015	20.9
2016	21.7