With this datapack you can change the vein size of all the ores of Minecraft

Tutorial:

Extract the zip file insiede a folder,

To modify the vein size of the ores, go to data/minecraft/worldgen/configured_feature and change the size value in the desired JSON files (The maximum size is 64).

To modify the vein chunk of the ores, go to data/minecraft/worldgen/placed_feature and change the size value in the desired JSON files (The maximum size is 64).
After that, select the file 'pack.mcmeta' and the folder 'data' and create an archive with the .zip extension, and put the zip file inside the folder 'datapack' of your world

{
  "type": "minecraft:ore",
  "config": {
    "size": 17, //change this value
    "discard_chance_on_air_exposure": 0.5,
    "targets": [
      {
        "target": {
          "predicate_type": "minecraft:tag_match",
          "tag": "minecraft:stone_ore_replaceables"
        },
        "state": {
          "Name": "minecraft:coal_ore"
        }
      },
      {
        "target": {
          "predicate_type": "minecraft:tag_match",
          "tag": "minecraft:deepslate_ore_replaceables"
        },
        "state": {
          "Name": "minecraft:deepslate_coal_ore"
        }
      }
    ]
  }
}

{
  "feature": "minecraft:ore_coal",
  "placement": [
    {
      "type": "minecraft:count",
      "count": 30 //change this value
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:height_range",
      "height": {
        "type": "minecraft:uniform",
        "min_inclusive": {
          "absolute": 136
        },
        "max_inclusive": {
          "below_top": 0
        }
      }
    },
    {
      "type": "minecraft:biome"
    }
  ]
}

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