Year	Number of
1990	22.02
1991	21.62
1992	21.03
1993	20.35
1994	19.72
1995	19.24
1996	19.02
1997	19.01
1998	19.16
1999	19.38
2000	19.63
2001	20.25
2002	20.93
2003	21.68
2004	22.48
2005	23.33
2006	24.45
2007	25.61
2008	26.87
2009	28.21
2010	29.65
2011	31.38
2012	33.28
2013	35.34
2014	37.53
2015	39.85
2016	42.34