We know that Borg designations for species went up to the low five digits. But we also know that the Borg gave designations to species they had not (yet) assimilated, so that tells us nothing.
Nothing was revealed about how the Borg began. When they began has always been pretty vague, beyond the general idea of them being old. In TNG, Q and Guinan had suggested they were "hundreds of millennia" old, whereas a single VOY episode, "Dragon's Teeth", had some local players go and claim that the Borg hadn't been that much of a nuisance mere 900 years prior. But that's not a contradiction, because VOY also showed that the Borg were secretive, and could lurk in the shadows just about everywhere in the galaxy without being observed. The Borg might simply have been in a passive mood around those locals at the time.
"Hundreds of millennia" is old but not ancient: the various shows have made references to civilizations that have existed in more distant past. We don't yet know of another civilization that would have lasted for hundreds of millennia, though... Except perhaps the Bajorans. And they seem to have been strict homebodies rather than starfarers. Perhaps those two are the the only ways to survive that long: be unnoticeably tiny, or be unnoticeably stealthy?
Timo Saloniemi
