How many meters of cable can be connected to each input of the contacts interface with two independent inputs (20490, 14490, 16940)?

You can connect a maximum of 30 meters of cable to the magnetic contact plus 30 meters return cable: the total for the connection sections for each input must not exceed 30+30=60 metres of wire. It is good practice to pass the wires along separate channels to avoid possible induction interference from powered cables.