They ostensibly destroy all the shafts they find, so I doubt they know how many tunnel entrances there are but they might have broad estimates.
As for the hostages, I don’t think they’re telling anyone their plans.
I’ve been thinking about this. Do they blow up tunnels without entering them? That would mean blowing up potentially valuable sources of intelligence, along with the knowledge of intersecting tunnels.
I understand it’s very dangerous to enter, but hard to imagine they simply blow it up and walk away.