Year	Number of
1990	3.66
1991	3.83
1992	4.01
1993	4.2
1994	4.4
1995	4.61
1996	4.82
1997	5.05
1998	5.3
1999	5.56
2000	5.87
2001	6.19
2002	6.52
2003	6.88
2004	7.26
2005	7.69
2006	8.16
2007	8.63
2008	9.09
2009	9.54
2010	10.02
2011	10.72
2012	11.53
2013	12.41
2014	13.33
2015	14.3
2016	15.36