In the daytime, all mobs spawn underground. Therefore, if 20 mobs can spawn in your chunk, all 20 would be in your cave, the only place where the light level is low enough.

In the nighttime, some mobs spawn on the surface. Therefore, if 20 mobs can spawn in your chunk, not all of them would be in your cave, as the light level is dark enough on the surface.

Therefore, the song "Don't mine at night" is wrong, and the creator should be sued for making me lose my diamonds ^/s