There's no practical limit to the number of DFSORT symbols you can specify in SYMNAMES. At some point, there might not be enough storage available to save off the symbol information, but that point would require an extremely large number of symbols, so it's doubtful you'd run into that limit in any real situation. I just tried 100,000 symbols of the form:
and it worked just fine. I'm sure I could do more but that seemed enough for a practical test.