It really depends on what you want. My campaigns "First Season" (10 missions) was build around three Lines of Operations. One was Baraban on Greyhell Front, second one was Castobel in the route of Hivefleet Dagon and third one was Inquisition investigating some ancient ruins. Then I basically gave the Kill Team more requests of assistance than they could ever handle, forcing them to choose and prioritize which missions they would prosecute based on their characters opinions and morals. This lead to a lot of nice roleplaying as the Kill Team members discussed the merits of each operation. This style works well if you want to throw a lot of different opponents on the players.
Another styles are to focus the campaign around a specific location or a specific enemy. You could, for example, write a whole campaign about saving a single world (single world, 5 marines… sounds okay) or about stopping a specific enemy (like one tendril of Hivefleet Dagon). In this case the campaign would probably have a single world or some other important location as a sort of "metacharacter" around which the campaign would rotate.