Year	Number of
1990	4.59
1991	4.7
1992	4.83
1993	4.97
1994	5.13
1995	5.3
1996	5.5
1997	5.71
1998	5.96
1999	6.21
2000	6.49
2001	6.83
2002	7.19
2003	7.57
2004	7.97
2005	8.4
2006	8.75
2007	9.11
2008	9.49
2009	9.88
2010	10.27
2011	10.59
2012	10.94
2013	11.3
2014	11.69
2015	12.12
2016	12.6