21 - shin
ש can be pronounced either /sh/ or /s/. In vocalized texts, a dot over the ש indicates its pronunciation: שׁ (with a dot on the top right) is /sh/ whereas שׂ (a dot on the left) is /s/.
Without nikud, the distinction between שׁין /shin/ and שׂין /sin/ is based on prior knowledge of the word. Therefore, some texts, though generally not vocalized, still mark the שׂ (the less common of the two) with a dot, to distinguish it from the שׁ.
שׁ and שׂ are two different letters.
Two words may look identical, but if one is spelled with a שׁ
and the other one with a שׂ,
then they are pronounced differently and have different meanings.
For example: