The latest episode of The Flash seems to indicate how fast Barry Allen (Grant Gustin) can actually run. The Scarlet Speedster's top speed has long been a topic of discussion, but not something that's been set in stone.
After gaining his metahuman powers in the pilot episode of The Flash in 2014, Barry Allen has fought hard to train his powers and build up his ability to run at super speed. With the help of S.T.A.R. Labs and the advice of other speedsters, Barry has succeeded in becoming the Fastest Man Alive. The challenges that Barry has faced over the course of the series has forced Barry to reach new limits. Barry has had to contend with speedsters who could easily outrun him, like Reverse-Flash and Zoom, but Flash has managed to surpass and defeat them all.
In The Flash season 6, episode 2, titled "A Flash of the Lightning", Team Flash deals with a metahuman who emits ultraviolet light. Barry experiences difficulties dealing with her powers because of how fast her attacks are. The reason for this is revealed when it's mentioned that ultraviolet light is "80 times" faster than Barry's top speed. All electromagnetic rays travel at 186,000 miles per second, so by this calculation Barry should be able to reach around 2,325 miles per second.
This number hasn't been confirmed. One problem with this idea is that in The Flash's season 6 premiere, Barry worked up enough speed to break out of a black hole, which should have required him to exceed the speed of light. So despite some inconsistencies, it's clear that Barry has come far in six seasons. In the season 2 episode, "Trajectory", Barry's top speed was 2,532 miles per hour, which means that Barry's speed has increased exponentially in four seasons.
As fast as Barry is now, this may not be his limit. New enemies and obstacles have put in Barry in situations where he has to run faster than ever, so it's possible that Barry can break his record with enough effort and training. After all, the comic book version of Barry Allen (and Wally West) are several levels about his TV counterpart, so there is room for growth. That much growth may become necessary, especially in season 6 with "Crisis on Infinite Earths" looming. Since the Monitor (LaMonica Garrett) insists that a sacrifice from Barry will be needed to save the multiverse, the time may come for Barry to somehow reach new heights.