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It isn't possible -- custom columns don't know the value of other book records.
You could access those records with a plugin or a calibre-debug.exe script which writes static data to a custom column -- this has been used before, to good effect, for e.g. formatting series with one leading zero if books<10 and two leading zeroes if 10<=books<100
But I'm not sure what you are trying to accomplish, nor how.
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