Is the only known/canon way for someone to become an officer in Starfleet is by way of the Starfleet Acadamy campus in the former Presideo in San Francisco, California, United States, North America, Earth (Terrra), Sol System?
If not, why haven't others been shown onscreen?
If so, how does Starfleet keep up with manpower requirements? How many officers would need to be commissioned per class to meet manpower requirements?