ReadEdenFoundation is the beginning. Why does one edition call it volume six?
Start with Foundation
If you want the story that made the series famous, begin with Foundation, then read Foundation and Empire and Second Foundation. That is the original trilogy in publication order and the clearest first route.
Where does the number six come from?
Asimov later connected the Foundation novels with his Robot and Galactic Empire books. Some publishers number the combined universe chronologically. Earlier events then occupy the first positions, pushing Foundation further down the list even though it remains the original entry point.
What comes after the trilogy?
Asimov returned with Foundation's Edge and Foundation and Earth, which continue beyond the trilogy. He then wrote two prequels, Prelude to Foundation and Forward the Foundation.
Publication order keeps the discoveries intact
The prequels happen earlier, but they were written with knowledge the original trilogy reveals gradually. Read the seven Foundation novels in publication order on a first visit. Explore the larger Robot-Empire-Foundation chronology later if connecting the universes becomes part of the fun.