15 villagers is probably about as many as the farm can comfortably support. If you make any more baby villagers before you get to Level 3 of Harvesting, you're almost certain to have significant issues with your food supply. 15 villagers can eat a lot of food! I never go above a population of about 12 until I have Level 3 of Harvesting.
_________________________
Barbara
Unicorn
Last Day of Work