Set the figure in the specification rather than leaving it to site, because a contractor asked to decide will choose zero.
The good news is that standard core counts do much of the work for you. Control cables are made at 2, 3, 4, 5, 6, 7, 10, 12, 19, 27, 37, 44, 52 and 61 cores, not at every number in between. Need eight cores and you're buying a ten anyway, so the spares arrive free. The outside diameter barely moves either: on the 1.5 mm² construction, going from 7 to 10 cores takes the cable from 14.5 mm to 18.0 mm.
Terminate the spares onto the terminal block at both ends and mark them on the drawing. A spare coiled inside a gland box is a spare nobody will find. See the core-count planning guide and the control cable range up to 61 core.