Looks like the Virtue bridge will have a max of 9 bridge officers. If you don't have the max amount of bridge officers or not enough of a certain career, such as engineering, to fill all the stations, basic NPC officers will appear instead of your own.
Example: if the bridge you are using can hold 9 boffs and you only have 6, the other 3 locations will have NPCs at the stations. Or if the bridge holds 9 boffs and has 3 engineering stations and you have 12 boffs but only 2 engineers, the third station will be filled in by an NPC.
I could be wrong on some things though. But the bottom line is that each bridge will only have so many officers stations when you enter your ship interior, no matter how many boffs you have.
I went to the Delta Quadrant and the Kazon stole my stupid T-shirt. Also, my pet hamster that died when I was a kid is now a Kobali.