A simple shell script to create a ramdisk under macOS. Tested under Monterey 12.1. Note that it takes two shell arguments, first the name of the ramdisk and secondly the desired size in GB.

mkramdisk

#!/bin/sh

diskName=${1:-'RamDisk'}
gigsRequested=${2:-4}
bytes=$(expr 2097152 \* $gigsRequested)
echo "Starting to make $gigsRequested GB ramdisk named $diskName..."

if diskutil erasevolume HFS+ $diskName $(hdiutil attach -nomount ram://$bytes); then
    open "/Volumes/${diskName}"
    echo "Success!"
else
    echo 'Error: Unable to make ramdisk'
    exit 1
fi

Don't forget to chmod -x mkramdisk to make it executable.

Usage:
ramdisk 'RamDiskNameHere' 8