It is in my understanding that the region that defines a mob limit is being reduced around a spawner in the new dev builds from 17×9×17 to 9x9x9. This will allow it to be easier for players to make drop traps to continuously farm skeletons and other mobs for their resources to an infinite amount.

My idea would be to take advantage of the block entity system to add a defined limit (for example, 20,) mobs that can be spawned from the spawner on a countdown then once it reaches 0 have the spawner self destruct or delete itself with a smoke or explosion particle effect on it's destruction, ultimately limiting how much players can farm from mob spawners.

For adventure maps creative mode created spawners could possible either via cheat item spawn have a much higher or infinite amount set for them, while natural dungeon spawners could have a set or random amount they can possibly spawn before destruction.