I have single The Perfect Server - Debian 9 (Stretch) with Apache, BIND, Dovecot, PureFTPD and ISPConfig 3.1 installation on Upcloud's VM that has CPU: 4 cores & RAM: 8GB. I have about 30+ standard WordPress sites which are not high-traffic. Out of 7.8G RAM 7.8GB is used and 149MB free (see attached images). Is this normal? What would happen if I add more Wordpress sites? Are there any How-To guides anyone used to optimize memory on ISPConfig server?