Minecraft wiki entry for Ancient_Debris, associated text: There is an average of 1.65 ancient debris blocks per chunk[needs testing], with a normal maximum of 5. However, it is technically possible for up to 11 ancient debris to be found in a single chunk; adjacent chunks can generate blobs on the border with up to 2 blocks spawning in an adjacent chunk. As of Java Edition 1.16.5, on average they generate 1.56 ores per chunk. Given this number, and that there are 32768 blocks in a chunk (16 * 128 * 16), there is approximately a 0.004% chance that any randomly selected block will be an ancient debris block.