Like a Dragon: Ishin!

Just to add to the above guide, you can view how many Troopers you have enlisted by speaking to the Mission Receptionist at the Barracks and selecting Trooper Registry.

For me, it popped at 100/400. I had 97 and since I have more money than time, I just used the Enlistment option selecting the 5000 mon, which gave me 1. I simply did it 3x.

I suppose I could have ran around town and recruited 3 more but since I only needed 3, this was a time saver.

Just to add to the above guide, you can view how many Troopers you have enlisted by speaking to the Mission Receptionist at the Barracks and selecting Trooper Registry.

For me, it popped at 100/400. I had 97 and since I have more money than time, I just used the Enlistment option selecting the 5000 mon, which gave me 1. I simply did it 3x.

I suppose I could have ran around town and recruited 3 more but since I only needed 3, this was a time saver.

You need 112 troopers in total to unlock this achievement, the 12 you start with don't count. You can earn troopers via the enlistment option (the cheapest being 5000 mon each) or sometimes when getting into a fight on the streets you will receive a trooper upon completing the fight as long as you accept them. Some progress will be accumulated naturally through side content or by completing Battle Dungeons for the first time.