However, whether it's the beginning of your Bannerlord run or not, Looters are always a great source of renown. This isn't the only way to get large amounts of renown, but it's a good measure for how much you'll gain. If you have an army of 20 units and you go against one with 30, you'll get more than fighting a smaller group instead.
The best way to get renown in Bannerlord revolves around fighting groups of enemies with more units than you.