The Swedish authorities are keeping track. The Myndigheten för Samhällsskydd och Beredskap (MSB, or Authority for Social Protection and Preparedness) recently released the first report of its kind specifically tracking EV fires in Sweden and comparing them to combustion-powered vehicle fires and the results are clear: EVs are much less likely to catch fire.
https://www.motortrend.com/features/you-are-wrong-about-ev-fires/
Still haven't found a source explicitly stating parked car fire percentages, but they do say that most car fires are because of poor maintenance.