Ptkay,
Closed runway, per definition, is a runway once in use, present in older maps and approach charts, which shouldn't be used any more, for whatever reason.
Would you mind pointing me to that definition? ICAO Annex 14 ("Aerodromes"), from which the diagram showing how to mark "closed runways" is taken, does not define the term "Closed Runway" in the definitions part.
It only says
7.1.1 A closed marking shall be displayed on a runway or taxiway, or portion thereof, which is permanently closed to the use of all aircraft.
For a runway to be considered "closed" it is not required that it had been in use before.
As to how many "X" there should be, it goes on to say:
Location
7.1.3 On a runway a closed marking shall be placed at each end of the runway, or portion thereof, declared closed, and additional markings shall be so placed that the maximum interval between markings does not exceed 300 m.
So single crosses on each end are definitely not sufficient.
Reading on, a runway close to completion could be considered "temporarily closed", as the above only refers to "permanently closed" runways, and for those cases there is only a recommendation, not a requirement:
7.1.2 Recommendation.— A closed marking should be displayed on a temporarily closed runway or taxiway or portion thereof, except that such marking may be omitted when the closing is of short duration and adequate warning by air traffic services is provided.