Year	Number of
1990	18.36
1991	18.85
1992	19.46
1993	19.9
1994	21.03
1995	21.39
1996	22.56
1997	22.43
1998	22.11
1999	22.42
2000	22.89
2001	22.1
2002	22.53
2003	23.66
2004	24.77
2005	25.25
2006	25.59
2007	26.21
2008	27.39
2009	28.23
2010	29.08
2011	29.81
2012	30.13
2013	31.18
2014	31.94
2015	32.7
2016	32.84