Errrr... Was going to say I've completed about 5 but turns out it's way more then I thought.

Assassin's Creed 2 (PS4)
Assassin's Creed Brotherhood (PS4)
Darksiders Warmastered (PS4)
Crash Bandicoot (PS4)
Crash Bandicoot 2 (PS4)
Crash Bandicoot 3 (PS4)
Wipeout Omega (PS4)
Uncharted 4 (PS4)
Batman Arkham Knight (PS4)
Batman Arkham City (PS4)
Stardew Valley (PS4)
Need for Speed Rivals (PS4)
Valkyria Chronicles (PS4)
No Man's Sky (PS4)
Batman Arkham Asylum (PS4)
Steep (PS4)
The Witness (PS4)
Amplitude (PS4)
GTA V (PS4)

Super Mario 64 (N64)

Can't really complete the type of games I play on PC, stellaris for instance.

You have way too much time on your hands lol.

I beat 31 games this year. The games on bold are new games I've never beaten before, and the rest are repeat playthroughs. The games are also in the order I beat them this year.

• Final Fantasy XV (PS4)
• The Last Guardian (PS4)
• Bloodborne (PS4)
• Super Metroid (SNES)
• Super Castlevania IV (SNES)
• Super Mario 64 (N64)
• Recettear: An Item Shop's Tale (PC)
• Final Fantasy IX (PS1)
• The Last of Us: Left Behind (PS3)
• Trine (PS3)
• Tales of Xillia 2 (PS3)
• Darkstone (PC)
• Trine 2 (PS3)
• The Secret of Monkey Island (PC)
• Botanicula (PC)
• The Legend of Heroes: Trails of Cold Steel (PS3)
• The Legend of Heroes: Trails of Cold Steel II (PS3)
• Portal (PC)
• Portal 2 (PC)
• The Legend of Zelda (NES)
• Dark Souls III: Fires Fade (PS4)
• Rise of the Tomb Raider (PS4)
• VVVVVV (PC)
• Never Alone (Kisima Ingitchuna) (PC)
• Rogue Legacy (PC)
• The Angry Video Game Nerd Adventures (PC)
• Mandagon (PC)
• The Legend of Zelda: A Link to the Past (SNES)
• Gravity Rush (PS4)
• Hitman Go (PC)
• The Banner Saga (PC)

Honestly, I don't feel like he's telling the truth because the math doesn't fit unless he literally never leaves the house. To beat one hundred games he'd have to finish each one on average every 3.6 days. That means he'd have to BEAT those games in that short of a time frame. While technically it's possible, I just don't see anyone doing that. 

Most of his Steam games are like 3 hours long, so it's possible.

Very impressive. I didn't have a lot of time to spend on gaming this year because of my graduation but here is my list.

I will only list the games that I've completed. I played some other games as well but not that much.

Xbox One:

Final Fantasy XV (60-65 hours). Though I started it in 2016 I completed it in January so it counts for this year.

3DS:

Fire Emblem Fates: Birthright campaign (40 hours). I also started this game last year.

PC (Steam):

Nier Automata (40 hours) - Routes A,B,C,D,E

PS4:

Nier Automata (50 hours) - Platinum

MegaMan (2 hours)

MegaMan 2 (2 hours)

MegaMan 3 (3 hours)

Persona 5 (185 hours) - NG+ completed/Platinum

SNES Mini:

Super Metroid (3 hours)

MegaMan X (4 hours)

So, I've spent roughly 300 hours+ on gaming this year and completed 10 games though 5 of them were replays of retro games which are pretty short if you know what to do. I will probably be able to finish Wolfenstein: New Order and Doki Doki Literature Club by the end of the year but that's it.

more than 100 ;)

3DS:

• Ace Attorney 6: Spirit of Justice

PS Vita:

• Corpse Party: Blood Drive
• Demetrios - The BIG Cynical Adventure
• Full Throttle Remastered
• Steins;Gate Zero

iPad:

• 1979 Revolution: Black Friday
• A Normal Lost Phone
• Ace Attorney 4 - Apollo Justice HD
• Ace Attorney 5 - Dual Destinies HD - Episode 1
• Ace Attorney 6 - Spirit of Justice HD - Episode 1
• Ace Attorney Investigations HD - Episode 1
• Agent A: Chapter 3 - Ruby's Trap
• Batman Enemy Within - Episode 1
• Framed 2
• Her Majesty's Spiffing
• Hollywood Monsters 2 - The Next BIG Thing
• Kelvin and the Infamous Machine
• Layton's Mystery Journey - Katrielle and the Millionaires' Conspiracy
• Milkmaid of the Milkyway
• Monument Valley 2
• Paul Pixel - The Awakening
• Runaway 1: A Road Adventure
• Runaway 2: The Dream of the Turtle
• Runaway 3: A Twist of Fate
• Secret Files 2 - Puritas Cordis
• Syberia 1
• Syberia 2
• The Adventures of Bertram Fiddle 2 - A Bleaker Predicklement
• The Bunker
• The Detail (all 3 Episodes)
• The Walking Dead - A New Frontier: Episode 2

Switch:

• Super Mario Odyssey

Xbox One:

• Forza Horizon 2 - Fast & Furious
• Forza Horizon 2 - Storm Island & Porsche Expansion
• Forza Horizon 3 - Blizzard Mountain & Hot Wheels
• Sunset Overdrive - Season Pass
• Tales From the Borderlands (all 5 Episodes)
• The Walking Dead: Season 2 (all 5 Episodes)

PS4 on TV:

• Crash Bandicoot N.Sane 1
• Crash Bandicoot N.Sane 2 - Cortex Strikes Back
• Crash Bandicoot N.Sane 3 - Warped
• Game of Thrones (all 6 Episodes)
• Hidden Agenda
• Life is Strange (all 5 Episodes)
• Uncharted: The Lost Legacy

PS4 with PSVR:

• BattleZone  (PSVR)
• Bound VR  (PSVR)
• EVE Valkyrie  (PSVR)
• Farpoint with Aim-Controller  (PSVR)
• Loading Human - Chapter 1  (PSVR)
• Psychonauts in the Rhombus of Ruin  (PSVR)
• Resident Evil 7 biohazard  (PSVR)
• Star Trek: Bridge Crew  (PSVR)
• Summer Lesson: Hikari Miyamoto  (PSVR)
• The Last Guardian - VR Experience  (PSVR)

PC with Oculus Rift:

• Archangel  (Oculus)
• Elevator... To The Moon  (Oculus)
• Esper 1  (Oculus)
• Esper 2  (Oculus)
• Far Space  (Oculus)
• InCell VR  (Oculus)
• FORM  (Oculus)
• Portal Stories: VR  (Oculus)
• Quanero - The Relativity of Time  (Oculus)
• Rise of the Tomb Raider: Blood Ties  (Oculus)
• Star Wars: Trials on Tatooine  (Oculus)
• The Bellows: A VR Horror Experience  (Oculus)
• The Lab  (Oculus)
• The Price of Freedom  (Oculus)
• Tomb Raider The Dagger of Xian Demo  (Oculus)
• VR Kanojo  (Oculus)
• Wilson's Heart  (Oculus)

PC on monitor in stereoscopic 3D:

• Call of Duty: Infinite Warfare  (3D Vision)
• Doctor Who: The Adventure Games
• Doctor Who: The Gunpowder Plot  (3D Vision)
• Firewatch  (3D Vision)
• Metal Gear Solid 5 Ground Zeroes  (3D Vision)
• Metro 2033 Redux  (3D Vision)
• Rise of the Tomb Raider: 20 Year Celebration  (3D Vision)
• Yooka-Laylee  (3D Vision)

PC on monitor in 2D:

• Absent
• Ace Ventura
• Adam's Venture Chronicles
• Adventures of Willy Beamish
• Assassin's Creed Unity + DLC Dead Kings
• Batman: The Telltale Series (all 5 Episodes)
• Bear With Me: Episodes 1 + 2
• Bill & Ted's Excellent Adventure
• Blade Runner
• CAYNE
• Contradiction - Spot The Liar!
• Deep Space Waifu
• Forge: Chapter 1  (Loom 2)
• Forza Horizon 3 - Main Game
• Forza Horizon 3 - Blizzard Mountain & Hot Wheels
• Hollywood Monsters
• Igor Objective Uikokahonia
• INSIDE
• Ken Follet's The Pillars of Earth - Book 1
• Leather Goddesses of Phobos 2
• Life is Strange: Before the Storm (all 3 Episodes)
• Minecraft Story Mode + Adventure Pass (all 8 Episodes)
• Missing: An Interactive Thriller - Episode 1
• Mystery House
• Paradigm: Surreal Adventure Game
• Projector Face
• Puzzle Agent 2
• Sherlock Holmes 8 - The Devil's Daughter
• Snail Trek Chapter 1 - Intershellar
• Stories Untold
• Syberia 3 + DLC An Automaton with a Plan
• Tender Loving Care  (DVD-Version)
• The Adventures of Bertram Fiddle 1 - A Dreadly Business
• The Cat Lady
• The Inner World 1
• The Inner World 2 - The Last Wind Monk
• The Journey Down: Chapter 3
• The Journey of Iesir
• The Last Crown: Midnight Horrors
• The Last Time  
• The Lion's Song: Episode 1 - Silence
• The Little Acre
• The Walking Dead: A New Frontier - Episodes 1 + 2
• The Walking Dead: Michonne (all 3 Episodes)
• The Wolf Among Us (all 5 Episodes)
• Thimbleweed Park
• and a lot of Visual Novels, but I don't count them in

Don't worry, I believe you. A completition rate of more than 50% with a 4-digit game catalogue is impressive:

I'm only around 40%:

And in the same time other people binge-watch TV seasons, you can alternatively play a couple of short games each week and enjoy yourself even better.

It's all a matter of preferences how you spend you precious quality time at home (and thanks to mobile devices also outside home).

How does Steam count game completion? Will the game count as completed when you get a specific achievement or what? Is the game completion means getting all the achievements? 

I played a few VNs and other short games.

BotW, Splatoon 2, Xenoblade 2 and Odyssey took up too much time.
440 hours combined, that did not leave much time for many other titles.

10 - 15 would be my guess.