• 一人ひとり — one person
• 一番いちばん — number one / the best
• 一部いちぶ — one part / a portion
• 一致いっち — agreement / match
• 一回いっかい — one time
• 一行いちぎょう — one line
• 日本一にほんいち / 世界一せかいいち — number one in Japan / number one in the world

When talking about 一日いちにち as a duration, it means “one full day.”

But the first day of the month is read ついたち, a special reading from classical Japanese.