We already discussed this via email, but I'll post my reply here so others may benefit.

What you say about licensing is correct: developers should have a developer license (not a build license), and the 'build user' specified for the build engine (jbe) process(es) should have a build license (not a developer license).

In our own setup, we have only a single build user (user id "build"), which is used by all our build engines (several dozen of them, across 2 different projects). However, one might want to have different build users for different projects, particularly if the projects' access control is not public.

Similarly, if each team within a project is responsible for their own build machines, having a separate build user for each would allow you to have better control over the process permissions, e.g. disallowing team 1's build engine from taking requests from team 2's build definitions. Normally, however, this is managed by just configuring the build engine <-> build definition associations appropriately (and this is controlled by the process permission for modifying a build engine). But having separate build users would allow for even more control over the process permissions.