I'm not sure you can dismiss Shelob as easily as that. Tolkien writes:
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And as for Sauron: he knew where she lurked. It pleased him that she should dwell there hungry but unabated in malice, a more sure watch upon that ancient path into his land than any other that his skill could have devised.
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If the Dark Lord himself had thought so highly of Shelob as a safeguard for this passage, certainly Shelob would have attacked, no matter the size of the party. And having extra people around really wouldn't help, since, "those hideous folds could not be pierced by any strength of men, not though Elf or Dwarf should forge the steel or the hand of Beren or of Túrin wield it."