The latest episode of The Flash has confirmed that at least 221 Earths make up the Arrowverse. While the series has been largely vague upon the precise number of worlds that make up its extensive multiverse, this is easily the largest proposed number to date.
The idea of multiple Earths existing within the Arrowverse has been around since The Flash season 2. As Barry Allen and his allies battled the sinister speedster Zoom, they learned of the existence of Earth-2 (the world Zoom originally came from) and Earth-3 - the home of fellow Flash superhero Jay Garrick. Over the past few seasons, Flash's adventures have taken him to several other worlds and forced him into battle with a variety of characters from these alternate realities. Chief among these conflicts were his fight with the Nazi supervillains who ruled Earth-X and his fighting alongside the Flash of Earth-90 during the most recent crossover event, Elseworlds.
The newest number of Earths was revealed early on in The Flash season 5 episode 12, "Memorabilia." It was here that the dimension-hopping detective Sherloque Wells unveiled a device dubbed the Memory Machine, which can be used to enter into the minds of other people, similar to the movie Inception. While describing how the machine worked, Sherloque spoke of how he had used it in some of his earlier adventures, including a battle with Jervis Tetch - the Mad Hatter Killer of Earth-221, which confirmed that there are at least 221 Earths in the Arrowverse.
The exact number of alternate Earths has fluctuated wildly over the past few seasons. Originally, during Crisis on Earth-X, it was said that there were 53 Earths - 52 officially charted worlds and Earth-X, which few dimensional travelers wanted to go to as it was based around a timeline where the Nazis won World War II. Elseworlds expanded this number again, introducing Earth-90 - the setting of The Flash television series which ran on CBS for one year in 1990. No explanation was given for why this Earth had a number outside the designated limit, though it seems likely it was named after the year in which the first The Flash series premiered.
One possible explanation is that there are many more Earths that exist than are officially mapped out, and that the designations used by inter-dimensional travelers only note major ports of call. Another is that Sherloque Wells, being more widely traveled than most of the world-traveling characters we've met so far, may have personally mapped out several Earths outside of the original 52. It's also possible that Sherloque, who has been shown to be prone to exaggeration and something of a braggart in past episodes, may just be throwing a larger number out there in order to sound more impressive.
The most frightening possibility, however, could tie the question of how many Earths there are officially into the upcoming Crisis on Infinite Earths event planned for fall 2019. We already know that Earth-90 was destroyed as a result of the Monitor's machinations and that other worlds have fallen before the cosmic menace that he seeks to fight. Could there have been hundreds of worlds at one time before they began dying off because of the coming crisis? It's a question that will surely be addressed in future episodes of The Flash, as the countdown to Barry Allen's pre-destined disappearance continues.