minecraft-assets/data/minecraft/worldgen/structure_set/mineshafts.json

{
  "placement": {
    "type": "minecraft:random_spread",
    "frequency": 0.004,
    "frequency_reduction_method": "legacy_type_3",
    "salt": 0,
    "separation": 0,
    "spacing": 1
  },
  "structures": [
    {
      "structure": "minecraft:mineshaft",
      "weight": 1
    },
    {
      "structure": "minecraft:mineshaft_mesa",
      "weight": 1
    }
  ]
}