How much petrol in the reserve tank vauxhall corsa?

Corsa's do not have "a reserve tank" in the true meaning of the phrase.

On a vehicle with a reserve tank there will be a control knob/lever on the dashboard that, when the vehicle runs out of fuel, can be operated to allow the engine to draw from an "emergency" supply of fuel to allow you to get to a petrol station.

Most modern cars have done away with this, and instead have a warning light that comes on when there is about 2 gallons (10 litres) or fuel left, and then starts to flash when there is 1 gallon (5 litres) left.

This is less useful than a true "reserve tank" as a driver might fail to notice the warning light, but would be hard put to fail to notice they had run out of fuel and then pulled a knob out.