As of most recent update this is unobtainable.
If you have a previous save with less than 50 subjects in your kingdom would suggest you keep that save and load this up when the glitch is patched.
To unlock the achievement you will need to complete side quests in order to attract new subjects to Evermore.
The quickest way to get a lot of these subjects to Evermore is through Swift Solutions, you will naturally complete a lot of these errands through gameplay, you can also speed this up dramatically by building supply hubs (lumberyard, fish market, mine and farm) in Evermore. On completion each chapter you will be able to exchange tokens of gratitude (currency awarded for completion of errands) for side quests to attract new subjects, these are the quickest because you only have to walk up to the news subject and they will move to Evermore automatically and skips the fetch quest or defeat the necessary monster.
If you intend on optioning the platinum trophy you will need to complete the majority of the side quests to attract the 100 required subjects.
Side quests are very fast and straight forward, they can be completed at any time and locations are clearly marked on the map, you an use the nearest trip door and then make your way to the objective, this process can be even faster after you have the airship from Vector.
Side Quests involve fetch quests, skirmishes, killing tainted monsters or casting spells on NPCs who so desire.
You will need to do just over 50 to unlock the trophy, this can be extremely quick if you use the swift solutions method mentioned above.