Here's a quick datapack I put together to essentially increase the amount of mob spawns to 2x or more. (Default 1 shadow per mob)

It gives each mob a shadow, which is a duplicate of itself. If the shadow is not within 25 blocks of the player, it will despawn with a cool animation of it receding into the ground.

Do note that this pack may impact performance at high numbers.

To increase the amount of shadows per monster, use the command "/scoreboard players set shadow_amount sotv 1" and change the 1 to the amount of shadows per monster you want.

As of 1.0.1, monsters only spawn their shadows when a player is nearby, to improve performance, and enable higher numbers of shadows practically.