Year	Number of
1990	15.55
1991	15.83
1992	16.15
1993	16.14
1994	16.12
1995	16
1996	15.57
1997	15.18
1998	15.61
1999	16.29
2000	17.31
2001	17.79
2002	18.59
2003	18.64
2004	18.89
2005	19.63
2006	20.22
2007	20.57
2008	20.9
2009	21.44
2010	21.76
2011	22.25
2012	22.71
2013	23.02
2014	23.45
2015	23.91
2016	24.51