How strong an army is needed to take a tile in one strike. Or how to predict the amount of attacks..

Unclaimed tiles an be occupied in 1 hit. If the tile is occupied by an enemy then it will have a fixed durability value. This is 500. Your legions will have a Destruction value based on the number, level and type of troops in them. You can see the destruction value in the legion list (top left of each legion). You will need to combine attacks to gain 500 total destruction to take the tile.

It is possible to do this in 1 hit by filling the legion with Footmen. These troops have the highest destruction stat. If you have enough footmen of a high enough tier then your legion destruction value can exceed 500.