If you’re new to the game, one thing you should be aware of is the difference between reinforcing and stationing.
In the game anyone can reinforce anyone else. When you reinforce someone you send your troops to someone’s village and they stay there until you recall them. If you have all your troops reinforcing someone’s village it means you can’t make more as it still counts toward your troop limit. So if you have 500 troops in your village and you send them to reinforce someone, because that is the max number of troops you can have in your village and it still counts toward your troop total you cannot make more. Of course if some are killed you can make whatever number are killed again
Stationing in vassals is different to reinforcing. When you station troops in your vassal it doesn’t count toward your troop total, but rather to the vassal troop total. Vassals have a troop limit the same as your troop limit which is increases by researching command. When researched full it allows you to station 500 troops in each vassal. When you station troops in your vassal they cannot be recalled