All the actions of a player's army leaving the city and entering the field map are considered expeditions, including attacking monsters, participating in the assembly, attacking other players, collecting, transporting, scouting, garrisoning, etc.
The upper limit of the legion's troops is mainly affected by the level of the castle and also by the bonuses of technology, talents, and hero skills.
The total number of marching troops is the number of legions that can go on an expedition simultaneously.
The upper limit can be increased through technology and the VIP level.