There are 175 side quests in the game, 17 won’t appear until after the main story is completed, but you can grab the trophy after the story is done.
Side quests are given around the map, including in evermore, so look for the info bubble. Side quests are different from errands. Errands are given by swift solutions but you can trade Tokens of Gratitude to gain new citizens which will then give you a side quest to get that citizen.
All you have to do is complete 100 side quests and you are done.