In Shulchan Aruch Orach Chaim 432:2 the Rama says that the Minhag is to place bread in such a place that the searcher can find it. Both the Be'er Hataiv and the Mishna Berura say that according to the Arizal you should place 10 pieces and it is a Sod Godol.
http://hebrewbooks.org/pdfpager.aspx?req=14165&st=&pgnum=6
I am looking for a reason as to why we hide 10 pieces? Why not 7? Why not 15?