All depends on the power output, battery and even temperature conditions. Never has a flash been rated for its amount of output per charge, or has it? Usually, it's the battery that's rated.
if you use flash manual mode only, you can always refer to flash owner manual about the estimate shots for per set of batteries, it also depends on the what type of batteries you use.
if you are use ttl, there is no way to find out as the power outputs are varies for each shot.