Year	Number of
1990	18.51
1991	18.65
1992	18.72
1993	18.81
1994	18.85
1995	18.84
1996	18.85
1997	18.79
1998	18.39
1999	18.92
2000	19.09
2001	19.57
2002	20.17
2003	20.66
2004	22.52
2005	24.45
2006	25.59
2007	27.08
2008	29.2
2009	30.88
2010	32.54
2011	34.49
2012	35.9
2013	37.76
2014	39.52
2015	41.24
2016	43