Year	Number of
1990	13.2
1991	13.6
1992	13.72
1993	13.43
1994	13.15
1995	13.41
1996	13.16
1997	13.23
1998	14.13
1999	15.42
2000	17.77
2001	19.21
2002	21.05
2003	21.53
2004	22.21
2005	22.99
2006	22.78
2007	22.5
2008	22.12
2009	21.94
2010	21.74
2011	21.28
2012	21.35
2013	21.58
2014	21.68
2015	21.64
2016	21.96