One opinion...
Depends on the societal view of Psionics to a large part (already presuming the rules for Psionics are informative).
So in the Imperium post Psionic Suppressions, while the incidence of psi potential is the same the ability and desire to pursue it is far far far etc lower.
I may have goofed the odds and such below but it should be ballpark anyway. If anyone spots errors by all means correct them, I'll try to show my work to make the error points easier to spot.
Per CT let's start with the psi potential then. Even accept that the char gen rules inform for the entire population. That means everyone has potential (2D6), at least Psr2 and as high as Psr12. So we start with 100%.
So let's find an Institute and get tested and trained. Without testing and training that potential drops at 1 point per 4 years. And without training potential is nothing (except towards your desire to know the potential pool).
Our first wrinkle, we need a world with Pop 9+ (2D6-2) to have a chance of having an Institute. So our Psionic hopefuls have to be on the remaining 5.56% of worlds to hope to find an Institute. We've now cut our likely Psi population down to the same 5.56%, right?
But not all such worlds actually have an Institute. Just 5.56% to 8.33% of them. So let's take an average of that (because I'm lazy and rushed ;-) )... 6.945% and apply it to our 5.56% above. And we're down to around 0.386%... and we still need to actually FIND the Institute.
Finding the Institute (making contact with paranoid, persecuted, super secret society, in genuine fear for their very lives) is no easy task. We have at least a 27.78% chance (possibly higher if we have some Streetwise and Admin skill). Let's be generous and say our psi potentials average a skill bonus of +5 over the spectrum. So we have a 91.67% chance of finding the Institute (because our psi potentials are motivated). Still our current odds are lowered again, to about 0.354%
We'll also be generous and presume our psi potential aren't going to let a little thing like the Institute actually not being in downtown Metro and requiring a long trip to some remote part of the world stop them. Nor will we expect them to not have the Cr5000 for testing. It's a lot of creds, but not a fortune.
The training on the other hand is very costly (barring a gifted Psion getting a scholarship) at Cr100,000. I think that is going to reduce the psi potential pool significantly. In fact I'd say if you can't qualify for the scholarship you're unlikely to get training. So let's use that, Psr 9+ (27.78%) as the odds of getting trained. But being generous I'm not going to apply any aging reduction. So our psi potential pool is down to around 0.098%. Pretty slim. And we still haven't actually got any talents.
So, what about those talents? Well the odds again are more complicated than I care to get deeply into so let's just fudge a quick best case based on the 6+ roll for Telepathy taken first. That's odds of 72.22% and puts our psi potential pool at about 0.071% of the total population.
Around 4.75 million functional Psions on Earth at our current population of about 6.785 billion. That's a lot of freaking telepaths!
I'm uncomfortable with that kind of percentage, but as noted it based on PC generation. For PC generation it's not a bad number. For NPC or total population estimation I think you need to factor in a 1000 fold reduction at least. So just some 4,750 genuine telepaths on Earth.
Or something like that.