There are 21 notes in the main story and 9 notes in Justine included "Still Alive" achievement. All notes must be picked up in the same walkthrough. Else, it will not unlock an achievement as you picked up the missed note(s). See this wiki for a complete list of notes as seen under their notes' names or maps in any location pages. Diaries are optional unless you might if necessary.
https://amnesia.wikia.com/wiki/Notes_(The_Dark_Descent)
https://amnesia.wikia.com/wiki/Notes_(Justine)