Infinity, in all honesty. Whatever the story demands. I honestly could not provide an accurate account of all "PCs", "Characters", "NPCs", etc. I have never been into EU or Fandom, so all of my characters are "OCs".
In a character-driven RP, I'll prefer to have 1-3 (depending on what the GM needs) with a preference for 1 in order to maximize the amount of detail and nuance to the characterization and history. This is something I can do for every and any character, but an RP is a game, and there's only so much time in a day.
Other RP types have different numbers (such as RPing a nation for example, in which case I will usually make 5-10 hyper detailed characters in key roles, and make medium fleshed NPCs as the plot demands (this is much easier than it sounds with some world-building and lore to base it on, even if loosely).
For non-RP writing, the number of characters varies upon the story type with the goal of having a believable number for the plot's world (or slice of a world, or slice of a day or a minute, etc). Typically no more than 15 however.
If I ever do get that one story that wants to be told, then I'll probably need around 25-30 "full characters" I'd imagine. Not counting what one would doubtless see as "the protagonist" and background/npc characters. It's a very fleshed out world that I've spent more world building than writing in, so it's actually a case of limiting myself so I do not end up bloating the dramatis personae.