I have to ask, in all absolutely honesty, why exactly do you need 9 characters and what makes each of them completely and absolutely necessary beyond just the one for actually roleplaying. Do you realize 5 characters would be half of the current active group or so? See no problem here? And the rest, what NPC mentor and info role could they possibly serve that an apparently all-powerful dragon can't and why do you need 4 characters for this? I can understand an antagonist/villain NPC of sorts, god knows the PCs are going to need something to step on their way every once in a while.