And theres no real guideline for that. It's really just whenever you feel your villages you've already got are secure enough, and you believe you can hold your newly acquired village and your existing ones with the troops you already have. So that really there's more to it then nobles. Your surroundings, players and tribes around you impact how much too. A strong player or tribe, you'll probably want more troops before anything. Weak player or tribe, build troops and nobles and just take them over.