OK.

Historically, television seasons started in September, then ran until January where they would adjust their lineups. Some shows would have an early cancellation and new shows would be slotted in. The rest of the season would run until June. June was pilot season when a lot of shows would be pitched to the networks for their fall lineup. They would run reruns over the summer, then launch the next season the following September. This overall formula started to change when Northern Exposure was launched as a new series during that summer window, and as it was the only new show on TV, it drew a higher audience than expected and it took off.

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Generally, shows picked up from pilot to series would be on a contingent basis. They would have 4-5 scripts ready and start filming fairly immediately so they would have them ready for the launch in September, then most of the shows would be filmed just a few weeks before airing. The networks would only pick up the first half of an entire season (approximately 13 episodes), then renew the second half half based on ratings. If they did not have the ratings, then they would pick up another series and launch it.

Deep Space 9 was a second half series replacement with a launch date of January. It simply didn't have 26 weeks of programming before the summer schedule kicked in. Same thing happened with Voyager. Now, some of these shows were complicated by being syndication shows owned by the production company marketed to individual stations and networks, but the general framework was consistent.

Star Trek: TNG's second season was cut short by a writer's strike.

The math now is radically different. Seasons are much shorter and are now largely serial in dramas and science fiction, so they are more akin to the 1970's era Mini-series format. Some shows are specifically designed for extended second-run syndication. A television show still in production can generally also be marketed in the syndication market after 3 seasons, so there are ways to focus on that market. There are 13 weeks in a quarter year. Each week would have 5 daily episodes. So, a quarter worth of programming would be 65 episodes. So, producers generally want a minimum of 65 episodes over the first 3 seasons so that they can more easily slot into syndication scheduling. They'll lose a few days over the years, but that's a number they aim for. (If you look at the Disney teen lineup of shows like Even Stevens, Lizzie McGuire, etc, etc, you'll see they almost always stop right at 65 episodes, or just above that number. Lizzie McGuire was exactly 2 seasons, 65 episodes. Andi Mack is the only one I see coming in at 57 )