The following two sentences make sense; the first is where there's one system with one or more associated databases, the second is where there's multiple systems with one or more databases:

Backup the system's database(s).

Backup the systems' databases(s).

What convention should be used when there may be multiple systems? e.g.

Backup the system((')s(')) database(s).

The following two sentences make sense; the first is where there's one system with one or more associated databases, the second is where there's multiple systems with one or more databases:

Backup the system's database(s).

Backup the systems' database(s).

What convention should be used when there may be multiple systems? e.g.

Backup the system((')s(')) database(s).