Hello and welcome to Just Answer
She should eat several cups of food a day, which will vary based on the brand. Every name brand has feeding
recommendations, which will be listed on the bag/can, etc. Causes of not wanting to eat can range from sore teeth/mouth to gastrointestinal disease.
There are a few things you can try to stimulate her appetite:
Feeding slightly warmed canned cat or dog food will sometimes work.
You can also supplement her with some liquid sublingual vitamin B complex/B-12 from your local drug store. Typically they come with a dropper and you can give her 1cc under the tongue. It often helps stimulate appetite.
You can also try giving her some plain, active culture yogurt, 1-2 tablespoons. It will help ease any indigestion and aid in normal digestion.
If she continues to not be willing to eat, or starts to drink much less you should consider taking her in for a vet check-up.
I hope this helps
Please let me know if you have any follow-on questions