since we dont wanna put the spawning invisible sprite to an endless road(its tiring),we can try to find a way to put this spawner in specific distances using maths or some features in the game(i havent tried it).their gotta be a feature to do this. or we can use timer, i.e. after x seconds it spawns. but the latter is kinda inconvenient cause if player misses to kill the zombies in time, their would be too much zombies after x seconds.
i didnt see george zaharia post while i was writing this.
his post is the right answer.