In order to get to 7000 manpower you need to be mid to end game.

Requirements:
- Logistics tradition
- Infantry tactics tradition
- 4 Additional troops slots
- Military buildings
- Level 15 Marshal governing a military province

Logistics and Infantry tradition
The logistics tradition increases manpower by 15%.
Infantry tactics tradition adds 25% manpower to infantry and defense squads.
One of ur knights needs to reach level III with a trait in order to make the trait a tradition. Traditions cost gold and books.


4 Additional troops slots
A marshal starts with 2 additional equipment slots. To unlock the other 2 equipment slots your marshal needs the logistic III trait. With the tradition its easier to give your marshall the logistics trait. Equipment can only be bought in cities. Additional troops cost 8 workers and 2000 levies per slot.


Military buildings
To increase levies in a province you need upgraded military buildings in that particular province. These building are: Village militia, barracks, lord's castles, royal armory and royal palace. Higher levies means more troops in a squad.


Level 15 Marshal governing a military province
The higher the marshal's level the more troops you get. Assign your marshal to your military province and recruit peasants with your level 15 marshal in your military province. If no governor is assigned to the military province the amount of troops you will recruit is decreased.

Province with governor:


Province without governor:


Once you recruit peasant and reach 7000 manpower, the achievement should pop up.


A peasant army 7000+ strong is a reliable and cheap defence force but is very limited in offence due to supplies, siege equipment and strength.