Low breastmilk

I pumped each side for 15 mins and could get only 4 ml breastmilk. My son is 8 weeks old and I am giving formula milk on top of breast feeding him. How can I increase my breast milk. I breastfeed him every 3 hrs. Is my son is getting only 4 ml breast milk every time which I got while I pumping ? Worried whether formula milk will affect my son in future. Feeling guilty since as a mother I am not able to feed him to his need and I am not able to contribute for his health. I eat fenugreek seeds daily .