add an event handler to each target's init
for instance:
target1 addEventHandler ["killed",{playerScore = playerScore + 1}]
then, if the variable "playerScore" were over 20 you could end the mission or something. Or even display it.
EDIT: also, you could just use
this addEventHandler ["killed",{playerScore = playerScore + 1}]
and copy and paste into the inits of all targets