It's completely clear what you mean, so DB9 is fine. In most contexts calling it a DE9 will be more confusing.

If you ever find yourself wanting to order the connectors or backshells, it might be useful to know it's actually DE-9.

DB15 is the only one I have issues with. The company I work with has one container with "DB15" connectors (DA-15), and one with "DB15HD" (DE-15)