on the 757 there are 2 fire bottles. The two engines have a fire handle each.
Pulling the fire handle isolates the engine, cutting off: fuel, bleed air, hydraulics.
Twisting the fire handle left or right fires bottle 1 or bottle 2 into that handle's engine.
So, with an engine fire you want to dump all available extinguishing agent into the affected engine. With no engine fire, as a precaution, you fire a bottle into each engine.
Hope that helps.