First there is no such thing as an end-user developer license. Clients get access to development by adding the development granules to their NAV license. The comment I made on that other thread was in relation to production databases. You stated that you had 5 production (live) databases. Therefore I said you needed to have 5 NAV licenses.
Now the question becomes whether you need to add the development granules to all 5 licenses or if you could add them to just 1 and use that to develop in all 5 databases. The first thing that would depend on is whether all licenses have the same granules. If they do, my thinking is you could add the granules to just 1. As there is no limit on the number of non-production databases. But, as I said, you should verify this with your NAV partner.