Depending on which "spawn" you mean in that regard it's either this option
# Chance of a den respawning their boss when the den respawn time is hit
denRespawnChance=0.30000001192092896
# Chance of a den spawning per chunk
denSpawnChancePerChunk=0.04500000178813934
If you're updating from an earlier version, make sure to regenerate your configs so past artifacts can't mess with new options generating.