Restrict the maximum number of jobs running in the background while running in parallel.

run-parallel.sh

# This code CANNOT be run as a script.
# You should add this code to your shell rc file and use it as a shell function.
# This is because the `jobs` command can only get child processes.
# 
#  example: `for i in $(seq 1 24); do run-parallel sleep 6; done; wait`

function run-parallel() {
    MAX_JOBS=${MAX_JOBS:-6}

    while [ $(jobs -r | wc -l) -ge $MAX_JOBS ]; do
        sleep 1
    done

    $@ &
}