The Flash season 6 was cut short at 19 episodes. Three more episodes were initially planned, but ultimately weren't able to make it to air.
In The Flash season 6 finale, titled "Success is Assured", Team Flash faces off with Eva McCulloch aka Mirror Mistress (Efrat Dor), who is on the loose in their dimension and on the hunt for her husband, Joseph Carver (Eric Nenninger). Barry (Grant Gustin), Nash (Tom Cavanagh), Ralph (Hartley Sawyer), and large-scale battle at McCulloch Technologies. After getting her revenge, Eva succeeds in framing Sue Dearbon (Natalie Dreyfuss) for Carver's murder and becomes the company's new CEO. Meanwhile, Barry is still determined to rescue Iris (Candice Patton) from the Mirrorverse.
The Flash leaves the heroes in a less-than-ideal position, as they've just suffered a crushing defeat. While this works as an effective cliffhanger to end the season, this isn't the way the season was supposed to end. Since the plan for season 6 likely involved Barry and his allies defeating Eva in episode 22, season 7 would have opened up a whole new story, complete with new challenges for Barry to face. It's hard to say, though, if his Speed Force problem would have been resolved in season 6, or if that was always going to be reserved for season 7.
Either way, the plan for The Flash season 7 has changed, but what's not clear is how much it's changed. It should be expected for at least three episodes to be devoted to fighting Mirror Mistress, since that was the plan in the first place. But could that change? It's possible that with The Flash getting a fresh new season to start with, this storyline could be extended by a few episodes, and be over by midseason at the very latest. The arrival of Barry's next adversary may be delayed for a little later in the season than usual.
It's also worth pointing out that The Flash was going to have a Reverse-Flash in a new body.