One database can have multiple Moodles using it. However that means one set of tables for each Moodle instance. This is done by using a different table prefix during install, i.e instead of just one users table you can have mdl1_user, mdl2_users etc. This is straight forward and not problematic.

If you mean one set of Moodle code for multiple instances of Moodle that is called multi tenancy and is not available in standard Moodle.