There are 242 clues within the original game, excluding the DLC cases, equaling out to a total of 230 clues that must be found to unlock the achievement.
There are 320 clues within the Remastered Editions of the game (because all the DLC cases are now included in the requirements), equaling out to a total of 304 clues that must be found to unlock the achievement.
Replaying cases will give you a chance to search for missing clues, if you do not unlock the achievement by the end of the game.
Intuition and asking your partner can help direct you to clues or let you know if you still need to search for more clues.
A magpie is a bird, notorious for scavenging literally everything that they see.