Year	Number of
1990	21.51
1991	21.78
1992	22
1993	22.95
1994	23.48
1995	24.24
1996	24.33
1997	24.85
1998	25.21
1999	25.2
2000	25.21
2001	25.43
2002	25.82
2003	25.98
2004	25.99
2005	26.09
2006	26.44
2007	26.59
2008	26.56
2009	26.51
2010	26.8
2011	27.79
2012	28.26
2013	29.37
2014	30.49
2015	31.03
2016	31.77