The actual number and composition can vary with airline. The minimum for 61-200 pax used to be 3 of which 2 had to be Halon in the cabin. At least it was in EU-OPS, don't think it's changed with EASA. Then an additional Halon in the flight deck. So I'd say 4 total (3 Halon, 1 Water) as minimum.
Crash axe one in flight deck, if more than 200 pax an additional one in the cabin (Aft galley).