How to incorporate formula w breast milk

Right now I am exclusively pumping. My baby love is almost a month. She is eating about 4 oz every feed every 2 to 3 hours. I'm afraid I'm not gonna be able to keep up. How do I incorporate formula. Like if I do 1 or 2 a day. How much should I expect her to eat? And if I I Give her a formula bottle. When should I give her the next bottle if it's breast?