I would expect that would depend on the needs and requirements of your environment.

A secure type environment might require you to use a separate certificate for each database (even if all the databases on the server belonged to the same group / org / division / application,) while a less secure environment might be OK with all the databases being backed up using the same certificate for all.

Your best option would be to discuss which route to go, or if you even need to go down this road, with your boss and likely a company lawyer (hey! Lawyers *DO* have uses beyond boat anchors! :hehe: ) But start with your boss.