Year	Number
2004-01-01	18
2005-01-01	18
2006-01-01	18
2007-01-01	18
2008-01-01	18
2009-01-01	18
2010-01-01	17
2011-01-01	17
2012-01-01	17
2013-01-01	17
2014-01-01	23
2015-01-01	23
2016-01-01	21
2017-01-01	18