Year	Number of
1990	18.31
1991	18.8
1992	19.22
1993	19.67
1994	20.09
1995	20.62
1996	21.12
1997	21.65
1998	21.63
1999	23.82
2000	25.25
2001	27.13
2002	28.99
2003	30.68
2004	31.69
2005	33.1
2006	34.18
2007	35.74
2008	37.53
2009	39.55
2010	42.4
2011	45.43
2012	47.42
2013	49.32
2014	52.6
2015	55.63
2016	58.44