The best known way to create a Power is to make some mortals to believe in somebody powerful. But... what about making them to worship somebody who doesn't exist? Will it create a new entity - belief is power, after all? How many believers would be needed? And will it appear as a demipower, or only as a less powerful being, which will eventually grow to the level of a god?
Creating a new power
How powerful would such "Power" be? I imagine it just like Sougad the Lawshredder, or other denizens of the Harbringer House - ordinary beings with some amazing abilities, but unable to give spells to their followers or shape the Outer Planes.
To quote Ringo Starr, I'm certain that it happens all the time. Most gods were probably never mortal - they're personifications of concepts, hopes, and dreams. They don't necessarily start looking remotely humanoid, but they become that way over time. Who is to say how many legendary heroes actually existed in a remotely recognizable form? Maybe none of them.
Exactly how many believers are needed to create a god? It should be the same for any kind of god, whether based on a mortal or not. We don't know the exact number, and I don't think we should know. Overly precise numbers can be a monkey on the DM's back. The old Wizards of the Coast game A Primal Order suggested it was actually the world or plane that created gods using their own stores of cosmic energy, and the majority of people on the world or plane had to think of the entity as a god for the world to alter reality to follow suit. That doesn't work in a campaign world with many young regional pantheons, though. Still, it should probably be the majority of people in a given culture.
Why does this remind me of Adahn the Imagined in PS:T?
(Um... try your left sleeve again. And you had some money for me... And a magic weapon?)
To answer the question, though, I imagine that any of a number of things could happen. Maybe a confused new power appears, immediately unaware of itself and the multiverse around it.
Maybe a new Power appears, as a synthesis of the knowledge and beliefs of all of its newfound followers.
Maybe one of the worshippers meets their apotheosis, and becomes the new-found deity.
Maybe a dead god is reincarnated with a new name and new powers.
I'm sure there's others, and any of those could make a lovely plot...
Definite numbers of worshippers could be a problem, though. The Godsmen certainly believe in their own ascension, but none that I know of have actually done it (especially since it'd make one faction right - which would be a Bad Thing). The foundry hasn't turned into the Street of Small Gods, and I suppose the explanation might be that they all believe in their own apotheosis, and not so much in backing members one at a time until everyone ascends.
Perhaps what really matters is density of believers - so that a lonely cleric far from home gets some weakened spells, but the power has absolute control over their own kip. A concentrated, dedicated cult might just have what it takes to create their very own Power, but not quite enough to let it do much elsewhere.My rough guess would be that, if the belief is well focused, fairly dense, and popular enough to ship a gate town off the outlands or to send a whole planar layer to Mechanus, you might get a Power.