None, not for the render farm itself. You just need Arnold licenses for the render nodes. You need Softimage licenses for artist workstations; on the render nodes, you’ll be using xsibatch -processing -render to render (and -processing doesn’t take a Batch license for third-party renderers).

You wouldn’t need Maya Batch licenses for Arnold render nodes either. The Maya render/mayabatch command line tools won’t take a Batch license for third-party renderers either.