English slang quantities like a baker’s dozen, a score or a fortnight carry precise meanings that are rarely written down. Real quantities that English speakers in different countries say differently - not general slang, just the numbers underneath a colloquial word. Each one checked against multiple independent sources, not a guess.
Thirteen. The extra loaf dates from medieval bakers adding one to avoid penalties for selling underweight bread.
Twenty. It survives mainly in older texts and phrases such as "four score and seven years ago", which means 87.