Max plunder is based on your allies first attack someone and note down how much money you have gained then secondly buy allies with good Allie attack bonus over 300mill once u have done that attack somebody else and look at your money you've gained and u should have more the max p increases every time u get stronger from building forges and guilds so you need to keep buying allies to help increase your plunder and become successful in your attacks and gain more money
Actually if you won't money build cathedrals you'll get more money and always have atleast 5-6 guilds