Well that depends, if those 5 databases are all used for production then they should already have separate NAV licenses. A NAV license only allows 1 production database. If we are talking only 1 production database, and the rest are for development and testing, then I’d say 1 license would cover it.
If you had 5 production databases, each with their own license, I’m not clear if you could add the granules to just one and use that to develop in all 5. That’s a question I’d ask your NAV partner.