In addition to the specific noble assigned to and associated with a world, GT: Nobles assumes about 1 knight per 12.5 million people, about 1 baronet per 62.5 million people, and about 1 baron per 250 million people.
The figure in
GT:Nobles is
up to 1 baron per 250 million, implying that the actual number is smaller. Backwater worlds like Rethe probably don't have nearly their full quota.
The Solomani Rim supposedly has about one trillion people and about 300 worlds that are part of the Imperium. That would correspond to about 300 planetary nobles and about 80,000 knights, about 16,000 baronets, and about 4,000 honor nobles (mostly barons, but there might be higher titles, too).
High nobles, not planetary nobles. There would be up to 300 of them, but doubling up on titles would reduce that number. Countal, ducal, and archducal titles are not awarded for achievements, so there would be proportionally fewer of them among the honor nobles.
The Imperium supposedly has a population of about 11 trillion people, so the total number of knights, baronets, etc. (above and beyond the planetary nobles) is about ten times what one has in the Solomani Rim.
The figure I've heard is 15 trillion people.
That seems pretty reasonable to me.
It'll work for the Imperial nobility. In addition there could be 635 million planetary nobles[*] or the equivalent (for worlds without explicit aristocracies).
[*] This guesstimate is based on there being 508 peers in Great Britain in 1818 and a population in England and Wales of nearly 12 million in 1820.[**]
[**] Incidentally, the British peerage definitely does not follow the "one to five" rule. In 1818 there were 28 dukedoms, 32 marquessates, 210 earldoms, 66 viscounts and 172 barons, and I know (from a textbook I had back then) that the proportions were similar around 1970.
EDIT: I found the textbook I mentioned above (
Britain Today by R. Musman, first published 1973) In 1970 there were 31 dukes, 38 marquesses, 175 earls, 116 viscounts, 490 barons, 19 peeresses in their own right, 142 life peers, 19 life peeresses, 2 archbishops, and 24 senior bishops in the House of Lords. The population of the UK was 55.7 million. Or one member of the HoL per 53,000 people, about half as many as in 1818.
Hans