Complete Games

 

Some games have completion paths (like Legend of Zelda - go defeat Ganon and that's the endgame). Other games are melee based where you select a character and the teams face off against each other (like DOTA2 or Smash Bros) and you can play as many matches as you want but there isn't a big bad that you are defeating - you're more learning the characters and how to best face off against other characters while fighting little battles. Still other games are more open world where you can build stuff and again there isn't an end goal of victory, it's more engineering (I believe Valheim and Minecraft are examples of this). So if you're playing games in the latter categories, there isn't a completion. So I guess one of those is the only game you'll need. 

Text;
L: You shouldn't buy a new game until you complete the ones you already have.
M: WHAT.
L: Go finish DOTA2 and Valheim.