The answer to your questions are, yes, yes and yes.
Yes, you need multiple installations of moodle (but they can sit on the same machine).
Yes, you can use different 'subdomains' (I think you mean different directories in the same domain), eg. mysite.com/Moo1, mysite.com/Moo2, etc.
Yes, you can share resources. The different sites can all sit comfortably on one machine with one instance of MySQL and one instance of Apache running.
To get more than one version of Moodle running:
1. Copy Moodle into a different directory, and create a new Moodle data directory.
2. Edit config.php so that i) the new Moodle directory is set as the Moodle root, ii) the data directory is set to the new data directory. iii) the new Moodle uses a different MySQL database. iv) change the mysql user and password.
3. Go to MySQL and create the new database, and give permission to your new version of moodle to write to it.
4. Edit the apache configuration file, and tell it to serve up the new moodle directory. Also add an alias for the new moodle directory to 'mysite.com/Moo2' or whatever suits your fancy.
5. Browse to 'mysite.com/Moo2/admin' and set up your new Moodle site.
best of luck!
Hardware and performance
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