At most, only 2 bats can spawn in a player ticking area. I believe the limit was meant to be 5.

Alternatively, this may be working as intended, but in that case there is a bug (with no symptoms) in the spawn_rules/bat.json file in the 1.14 vanilla behavior pack. As seen here:

bats only spawn underground, so having a density_limit of 5 for surface spawns is pointless and confusing.

Steps to reproduce:

1. In a test infinite world with cheats enabled, locate a cave in a biome that spawns animals, such as a plains or forest biome. The cave must include a dark area below Y=63.
2. Stand on the surface above the cave.
3. Periodically execute the command "/testfor @e[type=bat]" and count the number of bats found.

Expected results:
Eventually, there are more than 2 bats.

Actual results:
No matter how long you wait, only 2 bats spawn. (There is a small possibility that one might fly out of the player ticking area, making room for another bat to spawn, and since the herd size is 2 there could be a total of 3, but it's unlikely you could get more than that.)