The Array has numerous psionic populations throughout. The estimation is that 1% would test as having some psionic capability, as an average throughout the cluster. All five major kingdoms have psionic programs, although the Shadar Empire focuses almost exclusively on countering psionic threats.
Sallumin
Sallumin is a special case, in that testing is freely available, but if you test positive for psionics, it comes with the Inherent Pledge. Essentially, all known psionics (including those visiting Sallumin) are assumed to be public servants. This is in keeping with the High Salluminic, which reveres natural psions and charges them with using their powers for the benefit of others.
The Shadar Empire
The Empire has a strict ban on psionics within their borders. Most Shadari citizens have a deeply ingrained fear of psionics, but subjects in the Empire are not all so antipathetic. Shadari outside of their borders can be put in situations where they have to co-exist with psions, but very few are comfortable with it.
Almerand
Almerand has psions, and uses them extensively, but testing is not widely available, so most low-level psions go undiscovered. Many psions work as Proctors, aboard Compact ships or in space ports, to administer the Volition Test.
The Dominion of Morath
The Morathi do not have a psionics program, but they do have apprenticeship programs. The Khans each have their own house Psion Master who tests promising youth. This program is open to all full subjects, not just the Unsaag and Glinde. The Khans tend to value clairvoyance above the other disciplines, and so pay the most for those positions.
Triene
In Triene, there is a sophisticated psionics program that spots candidates with a reasonable power level and enlist them to the military or service to the Aristocracy, and tried to suppress everything else. The exception is the Fareen, who have inherited psionic traditions dating back several thousand years. The wider Fareen population follows closer to the Array average, but the Houses of the Fareen have more than 20% psions, with more powerful psionic children being the product of incentivized selective inbreeding.
The Compact
Psionics are a tool of the Compact. Their agents have access to all the disciplines. Consider the psion career path in the Core book as the model of how the Compact recruits and trains psions.
Ushanti
The most psionic-oriented widespread culture is the Ushanti, who used amplification technology to enslave thousands of planets, but they have been pushed to the most remote sectors of the Heights, and are no longer considered a Great Kingdom.
Ghendo
The Ghendo are like wandering monks. They are violent, yet sworn from using any kind of weapon. This contradiction is the source of their strength. Using a weapon takes away their powers. If you are being tested by a Ghendo for psionics, you get +3 for your first discipline (awareness), and -3 for everything else after that.
Ghendo tend to make good ship’s crew, though they cannot be a gunner.
When a Ghendo gets their power, after a time it begins to subside unless they move to a different planet with life. They can then ‘re-root’ their power on the new world.