Assalamu aalikom

There are prescribed Sunnah prayers and there are general voluntary prayers. You can pray as many rak’ah as you want of the voluntary prayers.

Assalamu alaikum. I watch a lecture about the life of Juhaid Baghdadi.And it is said that he used to offer 300-unit prayer every day.How it can be so? I thought with all the sunnah prayer and night prayer it will not be so many. How can a person pray so that his prayers willbe so many? Thank you