I Read the "General overview on sensor calibration", but I don't understand if I should use one, two point or multi-point curve calibration. Which one should I use?

Single point: if you know the sensor is linear, and you know the slope. Determines the offset.

Two point: if you know the sensor is linear but not the slope, or you want to confirm the slope. Determines slope and offset.

More points: if you know the sensor is not linear, or you want to confirm the slope and linearity of a sensor that should be linear.

Indeed the more points the better, even for sensors where a single point would have done. At the very least you get a more accurate offset and a confirmation of the slope and linearity of the sensor.

For the MQ7, have a look at the data sheet to see what kind of slope you can expect and its range. Decide on your calibration points so they're spread out through the expected range. If not linear add a point or two in the range of your interest.