When you are in historical imagery and you look at the timeline, you can tell roughly how many historical images there are for the region you are viewing by the number of light blue bands on the timeline.
This location (Rio de Janeiro, Brazil) has a lot of historical imagery.
This location in rural Brazil, has very little historical imagery.
The only way we know of for automating the counting process is with the Google Earth API, and with the deadline for its end of life coming up this December we had better make use of it while we still can.
It will take a while to get the whole world done and decide on the best way to collect the data and the best format to show the results in, but we thought we would share some of our preliminary findings.
We have created a historical imagery density map for Spain and Portugal as seen below.
Strangely, over the oceans Google Earth sometimes reports no historical imagery but in other locations reports unusually high numbers. We are not entirely sure why this is, but it appears to be a bug in the way Google Earth reports historical imagery on the timeline. You can download the above KML file here.
We discovered that Google Fusion Tables has a remarkably easy way to create a heat map from a suitable KML file, as seen above. However the problem with high figures over the oceans drowns out the differences over the land. Nevertheless, there is a clearly more imagery over centres of population.