This is misleading IMO. The word “died” implies a verb, so my first inclination was to use “starben”.
Perhaps a more accurate English translation would be “How many dead?”
At the same time, is “Wie viele starben?” something that one would say?
Clozemaster cannot correct translations since they just use the translations by tatoabea 
Wie viele starben? is correct and common but not as common as Wie viele Tote?.