15-month old get their way

How often should a 15-month old baby get their way when they "disagree appropriately"?

For example, she is constantly (like 20 times a day) asking for food. Should we help her to "disagree appropriately" by giving her the words to say and then give her way? She also does this every time we are in the car (we got into the habit of giving her food when we get in the car, we learned our lesson).

What would you say is a good amount of yes's and no's to give in this situation?