One EA server should be fine but typically I would recommend that you install some 2nd level mirrors on other servers so they are not all pointing to 1 server for their updates. As long as all the clients on the network can communicate and be “Seen” by the server then you can manage all the clients from 1 location. However if you would need to create root EAS or if the server wouldn’t see all the clients then you would need to set up a Root EAS which would require a full SQL be installed so the database can be replicated.