Hello,
Symantec Endpoint Protection is supported both type of Network's whether your network is Workgroup or Domain.
However, in your case, 13 servers located in different center. Could you also let us know -
1) How many client machines would each of these servers be connected to?
2) Are these 13 servers carry independent SEPM machines or each connected via replication?
Incase, you choose Replication then you would require a Domain Network.
Here are Articles which may assist you to Install SEPM and SEP clients on Workgroup Network -
Best Practices for Central Deployment and Management of Symantec Endpoint Protection (SEP) in a Workgroup environment
http://www.symantec.com/docs/TECH91679
How to install Symantec Endpoint Protection in a workgroup environment
http://www.symantec.com/docs/TECH105662
Which Communications Ports does Symantec Endpoint Protection use?
Hope that helps!!