Here's a video on how to obtain ALL of the prologue achievements, and more! Links to the specific achievements, in order of when you'll obtain them, are in the video description. I didn't show battles as some are walked through by the AI, and most others are simply auto fought and won. The last one appears very difficult, but my game cut out, and I didn't complete the prologue.
As for this achievement:
1. You will have to play for a bit, until you get to the point where you take over Fort Dervingard.
2. After this, your army is allowed to be 20 individual units.
3. Get units, and when you've hit the point where you'll get 20, end your turn, so they officially become yours.
4. With 20 units, and none pending the need for you to end your turn, the achievement is yours!