Monitoring of Oracle Databases
OEM (Oracle Enterprise Manager or 'cloud control') is licensed under your Enterprise licence agreement. See:


Licensing of Oracle Enterprise Manager Cloud Control by Markus Flechtner
https://community.oracle.com/thread/4197719


So you need to install the OMS (Oracle Management Server) on one server, and an agent on each server that you want to to monitor databases or middleware like Weblogic.  You can use a single OEM installation to monitor your entire enterprise, irrespective of where the hosts are.  

Sometimes, due to firewall complications, organizations choose to have more than one OEM installation for ease of use, and sometimes as a backup in case OEM goes down.

However, Oracle licensing is complex and all queries are best handled by your Oracle account manager.