You could do this in onCityRazed in CvEventManager.py. To count things, just create a variable to store the count in. Python won't remember the value of the local variable when it exits onCityRazed, so the variable needs to either be declared as a global variable or imported into CvEventManager from your mod. If you are unfamiliar with these things, there is a lot of info out there. A good place to start would be Python.org.