David Diamond has written at least 11 symphonies; Persichetti, I believe,
did stop at his 9th, "Janiculum." (I might be mistaken about this.) Whether
Mennin's withdrawing his first two symphonies affects the case at all is
something I wonder, but I suppose not.  (And I thought Spohr _was_ supposed
to have written a 10th, now lost.) You both left out Roger Sessions.  ;)
Harald Saeverud wrote at least 9 symphonies; I don't know of a 10th.

Eric Schissel