How many teams in your world?
For me it varies from game to game. Sometimes the heroes are alone, sometimes they’re among dozens if not hundreds.
Licensed and other published settings tend to have a few teams around already as well as room to add more - indeed, they often feel somewhat crowded, especially if they include example NPCs in roles players might want to take up like the Avengers / Justice League style Big Team.
See New Orleans Superheroes for a recent example spun off from a Marvel idea, the 50 States Initiative.
That in turn was prompted by a return to the Heroes of Detroit game, which was about how Detroit specifically didn’t have any superhero teams before the Mayor pulled them together, and we never met another team from the rest of the universe.
Being just about alone makes the PCs feel more important and defining, but it needs a setting where rarity is built in.
The same is true of adventuring parties, Scooby gangs, Rebel Alliance cells, coteries of vampires and so on, but supers teams are often more visible across the setting. Dungeoneers might get into tales told by bards but it’s not as direct as footage and interviews on the evening news, while the others usually actively avoid fame for reasons of safety.
While I often tend to NPC bloat when GMing, I tend to go low when creating supers settings directly, partially because a smaller group of contemporaries means each one matters more - and partially because I struggle coming up with superhero names and power sets, and even more so with team names. This is why the New Orleans team still has no name two months later.