An adverb used in negative sentences to emphasize 'not a single one' or 'not any at all'; expresses that the number/amount is zero.
彼は宿題を一つも出さなかった。
He didn't hand in a single homework assignment.
昨日の試合で点を一つも取れなかった。
We couldn't score a single point in yesterday's match.
この店にはその本が一つも残っていない。
That book isn't available at this store at all.
その説明には納得できる点が一つもない。
There isn't a single convincing point in that explanation.