There is lots of good data on the number of executions carried out under color of law in every country in the world for almost every year, with a handful of cases like China, where the data is merely estimated.
There is far less easily available good data on the average time elapsed historically and in different countries between the offense, the conviction and the execution. Yet, this data shouldn't be terribly hard to secure for a good percentage of cases, and has the virtue that even a reasonably small survey sample has the potential to be a quite good estimate of the complete number in any given judicial system at any given time, because of the way a judicial branch bureaucracy works.
The data would seem to be a quite useful part of an index designed to measure the extent of due process in different criminal justice systems, or as part of an index designed to describe death penalties that are on the books in an empirical continuum basis, as opposed to an all or nothing basis. Countries with no formal death penalty, but high rates of in prison deaths of inmates, might be viewed a having de facto death penalties. Systems with low death penalty imposition rates, high rights of post-conviction death penalty vacations, and long post-conviction appeals processes might be viewed as de facto abolitionist. Each criminal justice system could be ranked on an apples to apples basis if the index was properly designed, with fewer countries ending up in a completely abolitionist stance. "Zero", would be the situation where life expectency in prison at every age, was equal to life expectency outside of prison, and there was no formal death penalty. A negative score might be assigned where life expectency in prison exceeded life expectency outside prison (a possibility in places like the United States where health care, shelter and food are legally guaranteed to those in prison but not universally available to those outside of prison who are under age sixty-five).