untar-in-parallel.sh

#!/bin/bash

# function to extract each tar file
extract_tar() {
  local file="$1"
  local folder="$(basename "$file" .tar.bz2)"
  mkdir "$folder"
  tar -xjf "$file" -C "$folder"
}

# get the number of CPUs
num_cpu=$(grep -c ^processor /proc/cpuinfo)

# create a semaphore to limit the number of parallel processes
semaphore=$(mktemp -u)
mkfifo "$semaphore"
exec 3<>"$semaphore"
for ((i=0;i<num_cpu;i++)); do
  echo >&3
done

# loop through all .tar.bz2 files in the current directory
for file in *.tar.bz2; do
  read -u 3
  (
    extract_tar "$file"
    echo >&3
  ) &
done

# wait for all background jobs to finish
wait

# clean up the semaphore
exec 3>&-
rm "$semaphore"

Thanks for your feedback @bobpaul! I really appreciate it :)

Let me introduce my untar version