there is no separate mode only for this, but can be scenario (mod) with stronger raiders attack, you can also check my mod : old gods where radiers attack are effect of killing "holy animals",

how many raiders should attack in your opinion?

Not sure if there is a mod for this, but it is pretty easy to create.
You can also try the "Ancient Warriors" scenario in the game, there are a ton of raiders in that one.

thank you, ill try the mods, idk how to make mods, otherwise ill do it my self x)

in my opinion they should be at least half the number of ur population, 20 pop means 10 raiders... i like the challenge in games, this game got me i like it, but i want the legendary difficulty, and i don't mean difficulty by starving, cuz theres no animals, i mean its a citybuilder with a little focus in tower defence, but i would love to se more of that tower defence, specially cuz of the era the game is about, normally or at least the games that i know of (Tower Defence) they are in modern eras