Year	Number of
1990	2.81
1991	2.99
1992	3.13
1993	3.25
1994	3.34
1995	3.37
1996	3.67
1997	4
1998	4.36
1999	4.78
2000	5.18
2001	5.21
2002	5.26
2003	5.43
2004	5.82
2005	6.5
2006	7.03
2007	8.14
2008	9.34
2009	10.47
2010	11.2
2011	12.43
2012	13.66
2013	14.9
2014	15.9
2015	16.6
2016	18.31