Year	Number of
1990	3.77
1991	4.01
1992	4.28
1993	4.56
1994	4.87
1995	5.22
1996	5.57
1997	5.96
1998	6.38
1999	6.84
2000	7.34
2001	7.72
2002	8.13
2003	8.56
2004	9.02
2005	9.5
2006	10.17
2007	10.89
2008	11.67
2009	12.51
2010	13.42
2011	14.25
2012	15.14
2013	16.09
2014	17.1
2015	18.19
2016	19.35