Mobs will rarely spawn when there is no light source nearby, even if it is the only place for mobs to spawn. If a player places a torch on the ground, nearby dark areas will start to spawn mobs.
Steps to Reproduce:
Boot Minecraft 1.14.4 Java edition.
Proceed to an open field with a wide-open space and no light sources nearby.
Wait until night time.
Throughout the night, make a note of how many mobs are spawning around you.
During the next day, place a torch nearby.
Throughout the next night, again make a note of how many mobs are spawning around you.
Results:
Notably fewer mobs will have spawned when there are no light sources nearby compared to when the torch has been placed.
Rate of Reproduction:
2/3
It seems like less mobs spawn when there's no lighted area, but thats because mobs have more space to spawn and they are more spread out. Minecraft keeps attempting to spawn mobs until the mob cap (70) has been reached. So when you restrict some of the area where mobs can spawn (in this case lighting it up), the mobs have less space to spawn, but Minecraft still attempts to spawn mobs until the mob cap has been reached and because of that it seems like there are more mobs, but in fact the mobs are just more crammed up.