moggie: can't speak for your Renault specifically- but the plenum chamber is *one* space that runs the width of the car an, as such, should only need *one* drain-hole. If this gets blocked, water rises in the chamber and can spill over into the heater air-inlet and then into the interior of the car. Where it ends up depends on several factors- water has a habit of going where you least expect it!
So , there may be only *one* hole to clear.