I’m confused, do you only want the results from relays themselves, or you want this info from all computers?
What is the final output you are looking for?
Also, I would recommend testing the session relevance in the Console Presentation Debugger or similar until you have it working exactly the way you want, then look at using it through the REST API from there. See here: https://github.com/jgstew/jgstew.github.io/blob/master/_posts/2018-10-29-Open-BigFix-Console-Presentation-Debugger.md
This should represent the maximum number of devices you should get in the results:
number of bes computers whose (agent type of it = "Native")
This would be to get the results from only Relays, not other clients:
number of bes computers whose (agent type of it = "Native" AND Relay Server Flag of it)
I think this is what @JasonWalker is suggesting you try:
(id of it as string | "UnknownID", name of it | "UnknownName", operating system of it | "UnknownOS", last report time of it as string | "UnknownLastReport", relay selection method of it | "UnknownRelayMethod") of bes computers whose (agent type of it = "Native" AND Relay Server Flag of it)
Something like this is required, because otherwise any computer who is lacking one of the things being queried will be silently removed from the results when using plural session relevance. This can be a good thing for efficiency reasons, but a bad thing if you are trying to include all possible items.