This is obviously impossible, no matter how charismatic you are. Najimi also frequently references doing things that would take more time to do than is possible.
The solution to this is simple, Najimi is living in a time loop. Anything he references that would take more time than is reasonable happened in previous time-loops, and she has so many friends because he already knows everyone perfectly from interacting with them in previous time loops.
- Actually it'd be around 312,500 new friends per year, ~856 per day (not hour), roughly 36 per hour, or about 1 every minute and 41 seconds. Obviously this is still impossible, but mathematically accuracy is important.
Joker's family and hometown are never shown. So headcanon: they're the same person. Joker looks somewhat older than Shousuke, being taller and with slightly longer hair, so it can be assumed that Persona 5 takes place later. This adds up with Shousuke being Shouko's younger brother who isn't High School age yet, unlike Joker.
What if Tadano's ability to read minds isn't just a fourth-wall breaking gag about reading thought bubbles? What if he can literally read minds? This establishes the possibility of unusual and extrasensory powers in their world, setting up the cognitive world as a feasible possibility.
Najimi's dressing as a girl in high school is shown to be taken as a whim from them, but based on their occasional sexual advances towards Tadano, and the fact that Tadano's overblown confession during their middle school years backfired on him in spite of his great efforts, made Najimi choose to dress as a girl to be with him and keep him company because Najimi knew that Tadano was hurt by this, but wasn't counting on Komi's presence.
This is somewhat evidenced by the fact that Najimi has a knack for interrupting Komi and Tadano's private conversations and personal moments, as these might be moments of jealousy instead of efforts to become the center of attention, as Najimi tries to pass them off as, and the fact that, initially, Najimi did not want to become Komi's friend. However, Najimi realizes that there's no way to put any blame on Komi for the situation, and becomes more tolerant of her presence around Tadano, as Najimi sees that Tadano is full of Joy when Komi is around. Najimi also learns that Komi, in spite of their initial impression of her, is an all around excellent person.
Another thing that happens to bring this out is the fact that even with Najimi's multitudes of friends at school, Najimi chooses to spend time at school almost exclusively with Tadano and Komi, having them close but keeping an eye on them so that they don't get too close to each other. This is only working partially, as Komi and Tadano are indeed becoming closer as time goes by, or Najimi is losing their grasp on them because Najimi recognizes that it's unfair to keep them from finding happiness together, as these are two people that they care so much about.
- Jossed when the character Rami Kawai was introduced in Ch.377.
The one throwaway gag where they're aware of Tadano's mind reading would imply that they possess the same ability, which could explain why they can make so many friends so easily; they can learn what the person they're talking to is like, simply by reading their minds. This could also explain why they were initially terrified of Komi; it's not because of Komi accidentally coming off as intimidating, but rather because she's immune to their ability, a trait they haven't seen in anyone else before. Similarly, they hang around Tadano and his family a lot because they're interested in Tadano's ability to read Komi's mind and is following him to learn more about how he does it.
As for why they need so many friends, it's possible they're some kind of Emotion Eater that feeds exclusively on emotions associated with friendship. This could be another reason why they hate preparing for exams; it's because preparing for exams means their friends wouldn't be free to go out with them, which means they would have less sources of emotions to feed on. Similarly, they often act as a Moment Killer between Tadano and Komi's tender moments because their relationship between each other produces a large amount of the emotions they crave, which draws them to the two.
Given how Najimi has made an impossible number of friends for our universe's passage of time, Komi and friends are living in a universe where time does not pass in the same way, making Najimi's five million friends as a sixteen-year-old plausible.
But the biggest tip-off to this is how people treat Komi. Many of her social anxiety traits overlap with autistic traits. In the real world, these traits (mutism, fidgeting, not properly responding to greetings, handshakes, and other social conventions, etc.) are seen as unfavorable and even disrespectful, and lead autistics to be outcasted, bullied, and scorned by their peers. But in Komi's case, her traits are not only accepted but worshiped. She is "othered" in the same way that autistics are in our universe, but rather than being the target of malice, she is placed on a pedestal. Therefore, the cast is living in a universe where neurodivergent traits are not seen as unfavorable or treated as problems to be fixed.
- He is insecure about his appearance, so he always wears a mask to hide his face.
- He suffers from incurable virus, so he wears that mask so others could not get sick.
- He is hiding his identity.
