Year	Number of
1990	18.92
1991	18.74
1992	18.67
1993	18.62
1994	19.53
1995	21.71
1996	22.1
1997	22.84
1998	23.44
1999	22.8
2000	22.68
2001	23.16
2002	24.35
2003	25.68
2004	25.13
2005	26.37
2006	25.72
2007	26.16
2008	26.57
2009	27.52
2010	28
2011	29.41
2012	30.06
2013	31.76
2014	33.03
2015	34.19
2016	35.99