dangkykinhdoanh You can, yes, but they'd need to be logically separated by a unique URL. For example, forum1.domain.com, forum2.domain.com etc. This is required because of the unique URL set in config.php
Databases can be on the same server, but obviously should also be logically separated, so db1, db2, and so on. In terms of the initial install, as long as the target directory is empty, the composer build should work fine.
If you can install one Flarum instance this way, then you won't need guidance to install the rest. Just treat them as logical instances, meaning that extensions installed for one instance do not impact another etc, and you'll be fine.
