I voted seven (or less), for the songs that are specifically the kind of epics that are Side-Length epics, the ones that were pioneered in the 70s by bands like Genesis, Rush, Yes, etc., with songs like "Supper's Ready", "2112" and "Close To The Edge".
There are longer songs, for sure, like "In The Name Of God", "The Ministry of Lost Souls", "A Nightmare To Remember", and "Breaking All Illusions" which have epic feels to them, but I'd say they're just longer songs in general, but because the band have so many of those 10-18 minute songs, it would feel disingenuous to the longer epics to call all of those songs epics as well and place them in the same category as ACOS, SDOIT, 8VM, etc.
Now, for other bands who don't have as many Side-Length Epics, like Big Big Train, I'd say their songs that are in the 10-18 minute range are definitely epics, especially given their epic narratives and structures. It's really all relative to a band and their output, but in the case of Dream Theater, I'd say it's just the seven that most of us have commonly agreed to calling epic. Now, if you go album-by-album, you could probably call a song like "In The Name Of God" the album's epic, being the longest song, and also the closing song (where many epics tend to be placed in prog albums). The same could be said for songs like "Trial Of Tears" and "Learning To Live".
-Marc.