In Andreas' case, the DirectAdmin server which he's using has shared php-fpm processes and, as far as I could tell, ondemand scheduling. This means that any sites which are completely idle do not have an active PHP worker.
With Enhance, every website is containerised and therefore it must have at least one running PHP process at all times. The security benefits of this are immense. However it does mean that a server full of idle sites will consume more RAM like-for-like than a server using a different PHP SAPI like mod_php or shared FPM as in this case. For Enhance, I would suggest budgeting 50-100MB of RAM per website which is not excessive in my opinion.
There is an exception; addon domains on Enhance will share a PHP worker with the primary website. Therefore addon domains which are idle will not consume any RAM. I would not recommend configuring websites in this way as you lose all the security benefits that containerisation brings but if you need to host a large number of sites on a VM with limited RAM then this is a possibility.
