Year	Number of
1990	15.81
1991	16.26
1992	16.72
1993	17.18
1994	17.65
1995	18.1
1996	18.56
1997	19.03
1998	19.49
1999	19.93
2000	20.45
2001	20.8
2002	21.29
2003	21.9
2004	22.41
2005	22.81
2006	23.15
2007	24.18
2008	25.11
2009	27.06
2010	28.81
2011	29.48
2012	30.63
2013	32.28
2014	33.98
2015	35.67
2016	37.21