why did zero say "so there are 52" in holes?

Zero is smart but he doesn't know how to read and write so Stanley is teaching him. Zero thinks that there are two different ways to write, capital and lower case meaning there are fifty two letters in the alphabet. Zero clearly thinks outside the box!

“Actually there are two ways to write each letter,” Stanley said. “So there are fifty-two,” said Zero. “Instead of twenty-six letters. There are really fifty-two. You can teach me ten letters a day,” suggested Zero. “Five capitals and five smalls. After five days I’ll know them all. Except on the last day I’ll have to do twelve. Six capitals and six smalls.”