Year	Number of
1990	16.99
1991	17.17
1992	17.35
1993	17.49
1994	17.61
1995	17.65
1996	17.79
1997	17.89
1998	17.77
1999	18.4
2000	18.87
2001	19.65
2002	20.37
2003	21.63
2004	23.12
2005	25.46
2006	26.82
2007	28.08
2008	28.98
2009	29.91
2010	31.34
2011	32.95
2012	34.89
2013	36.41
2014	37.92
2015	39.45
2016	41.46