a phrase meaning 13. It comes from the fact that in the past people who sold bread always added an extra loaf when somebody ordered a dozen (= 12) loaves, in case they might be punished for supplying too few.
BAKER’S DOZEN
Meaning of BAKER’S DOZEN in English
Oxford guide to British and American culture English vocabulary. Руководство по британской и американской культуре, Оксфордский английский словарь. 2005