What we do is:
(1) use a shared hosted provided database (in the case we use the MySQL database provided by Google Cloud SQL) - each Mautic instance will have its own database inside the shared MySQL instance
(2) use one VM instance with multiple /var/www folders (works like “apache folders”) - we need very few resources in terms of RAM and CPU to run each Mautic instance (and let us not forget that the database is outside this VM)
(3) finally whenever we want to share things between projects (e.g. layouts/templates) we setup a shared folder between the instances
This approach is good if you upgrade all instances at the “same time”, so that they all use the same technology stack (e.g. same php version, etc.).