Year	Number of
1990	3.82
1991	4.02
1992	4.13
1993	4.36
1994	4.48
1995	4.61
1996	4.32
1997	4.15
1998	4.06
1999	4.12
2000	4.11
2001	3.92
2002	4.3
2003	4.61
2004	4.72
2005	4.77
2006	4.94
2007	5.15
2008	5.59
2009	6.01
2010	6.36
2011	6.69
2012	7.14
2013	7.32
2014	8
2015	8.5
2016	9.18