Year	Number of
1990	6.9
1991	7.26
1992	7.36
1993	7.23
1994	7.08
1995	7.05
1996	7.66
1997	8.66
1998	10.11
1999	11.77
2000	13.49
2001	13.07
2002	12.55
2003	12.07
2004	11.83
2005	11.93
2006	12.78
2007	13.9
2008	15.28
2009	16.69
2010	18.1
2011	18.77
2012	19.49
2013	20.35
2014	21.26
2015	22.19
2016	23.18