There are 6 on the arrival shuttle, 1 in each of the supply closets. There are multiple masks and a tank dispenser in tool storage. There are several more masks plus firesuits and another dispenser in engineering. Atmos has a emergency closet too. All the heads and the captain have a Ecloset in their room. Medical has medical masks if you can get access to oxygen tanks. Fire station has masks and firesuits, also just needs tanks. There are two Eclosets at the end of the emergency shuttle arms. There are a few scattered about in the maintenance tunnels. There are I believe six medical masks in EVA storage along with a tank dispenser plus jetpacks. There are several Eclosets in crew quarters. There is at least one Ecloset in both of the security stations. I think I got them all.
So yea, for 12 players, there are more then enough freely accessible internals for everyone to have them, assuming something is going wrong.
Edit: A thought. First, fire station needs some form of air. As hilarious as it was, firefighters running in to combat plasma driven flame with no oxygen on their back is pretty dumb. Second, I think medical storage needs an oxygen canister, and an N2O canister, for refilling the anesthetic tanks. Third, under no circumstances should plasma research have easy access to internals. That should help keep noob griefers from bomb making. Fourth, same with security. It's rather fun watching security chase an atmos tech down and all pass out from CO2 he released earlier. Fifth, one preplaced oxygen tank should be full of N2O to deter early grabbing of internals. Second, Gas is funny.