How to pump more?

S.A.M • Mommy to twin girls!!! 🎀🎀

I’m solely pumping due to having preemie twins who were not latching for nursing. I am pumping every 3 hours (sometimes during the day I do 4 hours) and only still pumping the same amount. My girls are almost 3 weeks. Will I only ever pump the same amount? How do I get more?