The way I would go about it would be an own fewer objects conditon, then when it was met, have a treade cart randomly created somewhere. When they had the right number again, I'd re-activate the kill trigger, in case one of the creation spots was blocked.
Please, don't ask me how to do random stuff, there are many ways. If you have no clue, I recoment my util on timed random events, though there is also the bird method and patrol method. Timed events take twice as many triggers as random events, but they can't be manipulated using MARCO POLO, and there's much less room for error.
.:everything stated above is the truth:.