It varies on what lvl NPC, How far is it from the city(ies) your attacking from and What's your Hero's Attack.
To conquer a level 1 NPC
You need send a breaker wave of about Min 5k Warriors with 100 atk Hero, I advise you to use 5K Archers. then send loyalty waves of 1k warriors and archers.
To conquer a lvl10 NPC, you need to send a breaker wave of 100k Archers with Hero Min 200 atk. Then send loyalty waves of 10k warriors and archers.
If you want to know how to farm them which is similar but without capturing the NPC, see the related link below
Chat with our AI personalities
Build troops or attack npc
double click attack on a npc
You cannot. You can wait for the flat to naturally elevate as the server cycles otherwise the NPC is based on the level of flat you obtain it as... Flat 1 = NPC 1 Flat level 10 = NPC 10 so on...
It turns to NPC of the same level of the flat you built it on.
A flat is the only land which you can conquer and then build a new city upon.