There are a total of 50 secrets in Man of Medan that are carried between playthroughs and immediately saved upon pickup. It is not possible to grab all 50 in a single playthrough as some are in missable chapters, and some do require you to play in curator's cut or shared story modes. The list can be seen below:
Chapter: Uninvited Guests
[] - #30 Flight Plan
[] - #49 Anti-Aircraft Shell
[] - #40 Missing Lifeboat
Chapter: Ghost Stories (Curator's Cut / Shared Story)
[] - #20 Co-pilot Badge
Chapter: Abandoned Ship
[] - #26 Guard Duty Note
Chapter: Trapped
[] - #44 1947 Newpaper
[] - #28 Private Miller's Letter
Chapter: An Escape
[] - #14 Guard Duty Roster
[] - #45 Private Reed's Letter
[] - #7 Engineer's Logbook
[] - #11 Journal
Chapter: Caskets
[] - #23 Catering Staff Schedule
[] - #1 Diary
[] - #24 open casket
[] - #22 casket nameplate
[] - #35 small casket
[] - #13 locked casket
[] - #18 terrified corpse
[] - #39 electrocardiogram results
Chapter: Danny
[] - #41 Medan Hat
[] - #16 screaming corpse
[] - #32 dog tag
Chapter: Finding Friends *
[] - #21 hastily written message
[] - #43 infantry badge
[] - #4 locks Memo
[] - #19 Radio Operators Memo
Chapter: Ritual
[] - #38 chemical leak
[] - #37 Broken Wires
[] - #47 Cargo Ship Manual
Chapter: Pressure *
[] - #25 Ghost Rumours Report
Chapter: Glamor Girl *
[] - #5 Water Purification Plant Map
[] - #9 Man Overboad Search Orders
Chapter: Glamor Girl * (Curator's Cut / Shared Story)
[] - #15 Cargo Bill of Landing
[] - #31 Violent Incident Report
[] - #17 Manchurian Gold Test Results
Chapter: Distress Signal
[] - #50 Anti Aircraft Shell Casing
[] - #3 Gas Mask Instructions
[] - #42 Ship's Logbook
[] - #6 Nautical Chart
[] - #10 Navigator's Notebook
[] - #29 Radio Operator's Final Message
Chapter: Depths
[] - #48 Minutes of Meeting
[] - #27 Priavet Patterons's Medical Record
[] - #8 Engine Room Telegraph Log
[] - #34 Military Order
Chapter: Olson *
[] - #36 Casket Shortage Memo
Chapter: Matters of the Heart *
[] - #2 Matters of the Heart
Chapter: Matter of the Heart * (Curator's Cut / Shared Story)
[] - #46 Diplomatic Immunity Contract
[] - #12 ID Badges and Documentation
[] - #33 Toxins Research