How many masks were available in Chernobyl when the explosion?

Masks would not have helped, the entire area of the plant was highly contaminated with strong gamma, beta, and neutron emitters. These are hard to shield against. Most of the firefighters had already received lethal doses by the time they finished setting up to fight the fire, so they just kept working because it had to be done until they were almost ready to collapse. At that point they were taken to a hospital to see their families and die as comfortably as they could in the condition they were in. Most of these firefighters received near record doses, several thousand REMs.