Or the number of days since their birth? That’s the simpler explanation.
“Those apples are numbered” = “we know how many apples there are right now”
No, that does not make equal literal sense to what I said. Because days that are in the past are gone, we don’t have them anymore. We refer to moving through time as “killing” time or as “losing” time, in English we don’t tend to think of the past as something we currently have. The future is something we have or will have, the past is something we had and no longer have.