Personally what I've discovered is that when you send out troops to attack/ claim a subordinate city you have to at least match the cities power or more. When you attack your general has his own personal amount of power and the troops you have will also add to that. It's not the matter of how many troops you need to send as much as the amount of power you send with your general. Also on a side note. You need to be town hall lvl 11 to even obtain a subordinate city otherwise your just killing your troops.