Warning! SPOILERS for The Flash season 8.
The "Phantoms" episode of The Flash season 8 offered some insight into just how rare metahumans are outside of Central City in the Arrowverse and the mystery of how certain people get superpowers. Metahuman is the technical term DC Comics applies to ordinary humans who gain superpowers, distinguishing them from those who accomplish great things with magic or advanced technology and alien heroes like Superman and Martin Manhunter, whose special abilities are unique to their species. Most of the people with superpowers depicted on The Flash, including Barry Allen himself, are metahumans.
Questions regarding the origins and frequency of metahumans on Earth-Prime were raised early on in The Flash season 8, episode 9, "Phantoms," as heiress turned super spy Sue Dearborn (Natalie Dreyfuss) discussed her recent adventures with Iris West-Allen (Candice Patton). Sue mentioned that she was on her way to Coast City and hoped to catch a glimpse of the Coast City Phantom, who was reportedly the first metahuman in the city's history. Sue added that she had "run into quite a few metas around the world" while hunting the Black Hole crime syndicate during The Flash season 7.
When Allegra Garcia (Kayla Compton) asked how someone in Coast City could have superpowers, Sue explained that Central City did not have a monopoly on metahumans, despite probably having the largest population of superpowered people on Earth-Prime. Sue noted that there were many people all over the world who possessed latent metagenes and that all it took was the right set of circumstances to cause their powers to manifest. For Allegra and many other Central City citizens, this catalyst was the 2013 STAR Labs particle accelerator explosion, but potentially any life threatening trauma could cause an ordinary person to activate their metagene. Beyond being a primer for fans of The Flash who aren't well-versed in DC Comics' lore regarding metahumans, Sue's explanation also established just how rare metahumans are.
The Flash's setting of Central City has 14 million people, according to the episode "The Death of Vibe" from The Flash season 3. The Thinker claimed, in The Flash season 4 episode "Therefore I Am," that 2600 metahumans had been created by the STAR Labs particle accelerator explosion. Breaking down the numbers, this means that 1 in every 5,385 people in Central City (rounded up) developed metahuman powers in 2013.
There's no data regarding how big Coast City is on Earth-Prime, but one Green Lantern comic gave its population as 2,765,321. Presuming the Arrowverse Coast City is of a similar size, it would have developed 516 metahumans had it suffered a similar disaster as Central City. Thankfully, even in the Arrowverse, scientific accidents of such magnitude are incredibly rare. Nevertheless, it is hard to believe that a large city like Coast City wouldn't have recorded at least one metahuman since their existence became common knowledge following the STAR Labs accident in The Flash. This suggests that metahumans are far less common in the Arrowverse than earlier episodes of The Flash indicated.
New episodes of The Flash release Wednesdays on the CW.