If there's one thing most people know about Franz Schubert, it's that his most popular symphony is unfinished. If there's a second thing folks remember about Franz Schubert, it's that he only wrote nine symphonies. But if you look carefully at the list, you'll notice two oddities.
Symphony No. 1 in D major, D 82
Symphony No. 2 in B-flat major, D 125
Symphony No. 3 in D major, D 200
Symphony No. 4 in C minor, "Tragic" D 417
Symphony No. 5 in B-flat major, D 485
Symphony No. 6 in C major "Little C Major" D 589
Symphony No. 8 in B minor "Unfinished" D 759
Symphony No. 9 in C major "Great C Major" D 944
1) There's a symphony missing.
2) It's not the unfinished symphony.
Unfinished but included
The official "Unfinished" symphony (No. 8) incompletion wasn't because of Schubert's death. He wrote the first and second movement in 1822 (six years before his death), and just never got around to writing the last two movements. (most symphonies from this era had four movements). The first two movements were of such quality, though, that they were performed and have become part of the standard repertoire.
Unfinished and missing
The missing symphony is No. 7 is even more of a fragment than the 8th. composed in 1821, this E major symphony exists in sketch form, with a melody line and bass and counterpoint underneath. Unlike the 8th, all four movements of the 7th were written, and the first part of the first movement is fully orchestrated (the next step in the composition process).
The other other unfinished symphony
And there's another symphony missing from the list. In the final weeks of his life, Schubert composed a 10th symphony in a piano reduction score. That is, the music is written to be played on the piano, with notes indicating instruments for later orchestration. Like the 7th, it appears to be complete in sketch form.
With a significant amount of these compositions complete, it isn't surprising that musicians have been tempted to fill in the blanks.
Symphony No. 7's three co-composers
Schubert's 7th symphony was first completed in 1881 by John Francis Barnett, an English composer, and teacher. Famed conductor Felix Weingartner did his own version in 1934 and featured it in performance. The final version (to date) is by composer and scholar Brian Newbould, who extensively studied Schubert in order to make his completion as authentic as possible.
Symphony No. 10's two-and-a-half co-composers
The score for Symphony No. 10 was only identified as such in the 1970's, and Brian Newbould offered a scholarly completion of it. Conductor Pierre Bartholomeé revised Newbould's version with controversial results. And finally, composer Luciano Berio used the source material as the basis for his work "Rendering."
So how many symphonies did Schubert really write? In terms of completed works, seven. In terms of what's commonly performed, eight. And in terms of what might have been, ten.
But are these completions valid? It's difficult to say. Schubert may have revised the surviving material after working with it for a while. We can say a combination of instruments for a certain passage is likely, based on similar patterns in Schubert's other works. But we can't say it's exactly what Schubert would have done. There's always a possibility that he may have done something original and unexpected -- great composers are like that.
Personally, I think the sketches are complete enough that most of the resulting music is Schubert's. And I'm glad for the opportunity to hear these works, even in an adulterated form. They deepen my understanding and appreciation of this short-lived musical genius. (below is a performance of Schubert's Symphony No. 7 so you can judge for yourself).