Year	Number of
1990	4.61
1991	4.88
1992	5.18
1993	5.52
1994	5.88
1995	6.27
1996	6.88
1997	7.51
1998	8.24
1999	9
2000	9.82
2001	10.28
2002	10.74
2003	11.16
2004	11.58
2005	12.02
2006	12.54
2007	13.07
2008	13.69
2009	14.33
2010	15.01
2011	15.6
2012	16.26
2013	17.03
2014	17.83
2015	18.66
2016	19.51