New to breastfeeding

I had my son on Thursday night. I know I have milk now. I started to pump Sunday night. When I was breastfeeding my son was getting so frustrated nursing. When there is like no milk coming out. I know it take time. I had to start using formula too. He was not getting enough to eat. Yesterday he when all day without a wet diaper until 7pm. So he has mostly formula because of that. I made sure to pump a few time. I'm hoping I get more milk soon. How often sure I be pumping?