Year	Number of
1990	17.35
1991	17.87
1992	18.16
1993	18.58
1994	19.03
1995	19.25
1996	19.42
1997	19.56
1998	19.62
1999	20.23
2000	21.18
2001	22.1
2002	23.33
2003	24.53
2004	26.06
2005	27.37
2006	28.25
2007	29.84
2008	31.12
2009	32.74
2010	34.06
2011	35.4
2012	36.33
2013	37.41
2014	38.09
2015	38.63
2016	39.4