Year	Number of
1990	18.21
1991	18.79
1992	19.4
1993	20.04
1994	20.78
1995	21.54
1996	22.08
1997	22.67
1998	22.96
1999	23
2000	23.29
2001	23.34
2002	23.29
2003	23.59
2004	23.56
2005	24.02
2006	24.68
2007	25.12
2008	26.42
2009	27.13
2010	28.46
2011	29.92
2012	31.37
2013	32.72
2014	34.13
2015	35.63
2016	36.91