These are listed as each, only one is needed as a replacement.
Cross Section: Round
Size (Dash Number): 012
Durometer Rating: 70A
Actual Outside Diameter: 1/2″
Actual Inside Diameter: 0.364″ (9.25mm)
Cross Sectional Diameter: 0.070″
Temp Range: -76°F to +437°F
Any 012 O-Ring replacements can be used, but depending on material and Durometer rating (hardness) will depend on their life. Regular O-Rings (usually nitrile or Viton material) will also work, but silicone is the suggested material for a static seal such as this.