Yakuza: Like a Dragon

To unlock this achievement you need to complete 20 substories in the game. Substory is another word for side quest and overall there are 52 substories in the entire game.

There are three achievements involving substories, one is completing 10 substories, another for 20 (this one) and one for completing 40.

7 substories are mandatory, so you have to complete them to get through the game but the other ones you have to get on your own.

And don't worry! They don't become unavailable at all so you can do these at any point in the game!

Here is a list of all the substories in Yakuza: Like a Dragon: (Credit to IGN)
https://www.ign.com/wikis/yakuza-like-a-dragon/Substories

Have fun!