Year	Number of
1990	12.42
1991	12.72
1992	13.05
1993	13.39
1994	13.74
1995	14.18
1996	14.66
1997	15.3
1998	15.89
1999	16.52
2000	17.16
2001	17.12
2002	17.11
2003	17.28
2004	17.34
2005	17.43
2006	17.71
2007	17.87
2008	18.23
2009	18.86
2010	19.11
2011	19.57
2012	20.19
2013	20.82
2014	21.61
2015	22.39
2016	23.05