Imperians must not do pillages.They must stay at home , drink wine ,take care that their wifes are happy , ......certainly not go out fighing
(btw your answer : yes you need 20k soldiers to do max pillage to population of 100k ...at least so it is in old IO )
i would prefer to do nothing more nadiaa than drink, fornicate, stay home and take care of the wife and daughter, however there are some stupid, short.minded players that abuse their point advantage in the beginning and burn people to make 100k gold when there are nomad goldmines with 200k pop that dont fight back and you can make much more gold
as they no longer have a great point advantage or less points, revenge must be exacted and i must burn the stupid pillager over and over and over again, then kill the pillaging army.
it is under present analysis with my war council, recruiting mass troops of the light version to go and burn, burn, burn