You can see in many Starcraft 2 casts that there is a possibility to open a little window to see how many workers have been killed during the last attack. I checked most of the settings but couldn't find it. Anyone has an idea?
CTRL + R
You will need to get the number of units killed before the fight and then check the units killed after, there is no "per battle count".
You can find all about the replays and all the rest of the hotkeys here