Workflow Farm Topologies supports 5 server.

Workflow Manager farm leverages Service Bus behind . The Service Bus for Windows Server only supports a farm with 1 computer or a farm with 3 computers.
In order to have high availability you should have three servers No more, no less!
http://msdn.microsoft.com/en-us/library/windowsazure/jj193010(v=azure.10).aspx

Well after service bus 1.1 and workflow manager refresh 1.0 (or CU2 upade) Workflow Manager Farm can be support 1 or 3 or 5 servers .