novel with psi slavery

Question

Name the novel, 197x. Setting is a world of human(oid)s most of whom have the psi ability to enslave weaker others. (Maybe the word slave and its derivatives are not used.) Masters and slaves form a tree rather than a dichotomy; everyone delegates as much as they can.

The protagonist is an outworlder on a mission, maybe simply doing a survey. He wears the green robe of a native subculture who lack the power (or maybe they renounce it?) and are immune to it – by law or custom, I think, rather than by nature.

His host or primary contact is Jael Forty of Lant, the number being how many slaves he can control; it can fluctuate.

Answer 1

According to this Reddit post, this is The Mind Traders by J. Holly Hunter.

That was The Black, where men were punished for challenging minds more powerful than their own. The detective from Earth feared The Black more than any torture his own planet could conceive. But he had to uncover the sinister plot that threatened Earth and all its people.

...

Rigan society is rigidly structured based on mental power and the resultant ability to enslave people mentally. People are ranked based on how powerful they are, and that is represented by a number that represents the number of people they can Control (yes, with a capital C). Our protagonist, Jael, for instance, is a forty. He can Control forty people. It’s noted that all of the forty people he Controls are, in fact, thirties, which means that Jael has reached the limits of his power as a forty and is ready to take on the challenge to become a fifty.

Found with a search for "Jael Forty"