The CW is home to five DC Comics superhero TV shows, but the network's president says no more than four can air at one time. Before CBS and Warner Bros. TV merged their respective networks to form The CW, the channel operated as The WB - and one of its most popular programs was the Superman TV series, Smallville. Smallville proved that comic book shows could work, and that's why one year after it concluded in 2011, The CW launched its first comic book-based TV series: Arrow.
Arrow premiered in 2012 and became an astounding hit for the network, which is why they pursued The Flash spinoff series in 2014, and then DC's Legends of Tomorrow in 2016. While those shows rounded out The CW's core DC programs, they still had other comic book-related shows on the table, such as iZombie. Then, Supergirl moved from CBS to The CW in 2016 and became the fourth Arrowverse show. And while their latest venture, Black Lightning, isn't part of the Arrowverse, it's still based on a DC Comics property. So, to avoid oversaturation, and to prevent cannibalization of their other comic book properties, the network chooses to air only four superhero shows at any given time.
The CW president Mark Pedowitz told Bloomberg that, due to increasing competition from comic book-based TV shows, they will only air four superhero shows at one time, whether they are part of the Arrowverse or not. That doesn't mean he expects the superhero TV renaissance to fade away, though. He said: “The audience will tell you when the fatigue has set in. If you have a quality show or a fun show, the audience will stay with it.”
So far, the decision to air only four superhero shows seems to have primarily impacted Supergirl and Legends of Tomorrow, which are now sharing the same time slot. Legends of Tomorrow is currently on hiatus, but it will take over Supergirl's Monday night slot in mid-February when the Girl of Steel's series goes on break.
It's worth noting that while The CW has six live-action DC Comics TV shows on the air (and seven total comic book-themed programs, including Riverdale), they also air Constantine and Vixen on CW Seed, thus bringing their total DC Comics content to eight shows. However, only five are superhero-related programs: Arrow, Flash, Supergirl, Legends of Tomorrow, and Black Lightning.
That's a large number for one network. In fact, with all those programs under its belt, The CW has the most comic book-themed content of any network or service - on broadcast, cable, or streaming. But in order to avoid being labeled as a superhero network, they air other programs as well and restrict the number of DC superhero shows airing concurrently to just four.