Year	Number of
1990	12.06
1991	12.44
1992	12.78
1993	12.99
1994	13.09
1995	13.13
1996	12.76
1997	12.21
1998	11.98
1999	12.04
2000	12.2
2001	12.55
2002	13.2
2003	14.19
2004	14.51
2005	14.31
2006	14.56
2007	15
2008	15.57
2009	16.48
2010	17.53
2011	17.71
2012	17.84
2013	18.01
2014	18.39
2015	18.9
2016	19.33