Year	Number of
1990	4.52
1991	4.65
1992	4.75
1993	4.82
1994	4.89
1995	4.99
1996	5.17
1997	5.39
1998	5.7
1999	6.05
2000	6.44
2001	6.82
2002	7.25
2003	7.77
2004	8.31
2005	8.86
2006	9.48
2007	10.11
2008	10.8
2009	11.53
2010	12.3
2011	13.1
2012	13.95
2013	14.89
2014	15.88
2015	16.92
2016	18.09