Year	Number of
1990	19.55
1991	19.97
1992	20.45
1993	20.9
1994	21.41
1995	22
1996	22.49
1997	23.04
1998	23.65
1999	24.28
2000	24.93
2001	25.24
2002	25.57
2003	26.02
2004	26.49
2005	26.98
2006	27.17
2007	27.37
2008	27.61
2009	27.9
2010	28.25
2011	28.78
2012	29.38
2013	30
2014	30.62
2015	31.22
2016	31.87