When I thieve from master farmers, I can get around 20 herb seeds in an hour. I've gotten anywhere from 0-5 rannars in an hour, so you should expect around 2 per hour. My strategy is to bring 10 cakes, trap martin in the pig pen, and thieve until I need more food. Your numbers may be different depending on how you do it. Your only other options for getting rannars are as drops, using Manage Thy Kingdom, and the pest control seed reward, all of which are unreliable ways.