Under normal circumstances start with the one dedicated Back4 server and one Back4 instance that won't use more than 80% of CPU on the regular basis.

About adding more servers

In the GN4 version 1.4 you might add more servers, but you cannot divide tasks among them - all of them will concurrently work on all configured processes.

In the GN4 version 1.5 and 1.6 you can add more servers, and you can divide tasks among them - so one can receive wires, other can print, third can do the maintenance tasks, fourth can handle Exalead indexing and so on.

About more Back4 instances

In the GN4 version 1.6 you can create more Back4 instances on the same server, pointing to the same database, and split tasks among them - avoiding in that way that a problem on wire receiving affects printing and other background processes that run with another Back4 instance.

More Back4's can be installed on one server and point to different databases - that's useful in the cloud computing.

See also Multiple instances of Back4.