Year	Number of
1990	4.25
1991	4.32
1992	4.39
1993	4.46
1994	4.6
1995	4.85
1996	4.94
1997	4.81
1998	4.76
1999	5.01
2000	5.14
2001	5.38
2002	5.62
2003	6
2004	6.27
2005	6.35
2006	6.63
2007	7.02
2008	7.45
2009	7.55
2010	8.06
2011	8.29
2012	8.64
2013	8.97
2014	9.39
2015	9.46
2016	9.66