There are 242 artifacts in the game, but for this achievement you only need to collect 50.
Here is a breakdown of artifacts per level, followed by the total possible amount of artifacts collected. I saved the streets of Salem for last so you have access to all the abilities need to obtain them, however some in the streets are available from early on.
Apartments = 14 [14]
Church = 20 [34]
Police Department = 37 [71]
Cemetery = 28 [99]
Asylum = 25 [124]
Museum = 31 [155] *Are missable
Church [2nd visit] = 2 [157]
Town = 61 [218]
Judgement House = 24 [242]
This achievement is obtainable by the third level, the police department. Simply follow along the videos and it should be easy to obtain, I also added the streets of Salem video in case you collect some of the earlier available artifacts located there.
*Video credit goes to Harry94