I've run about twenty years of a campaign with eight multi-system NPRs, using 3 + 3 for the base / random.
There are 575 systems in the game, with 10 discovered by my conventional player race. About 80% of those are starting systems, with the rest discovered post-start.
There are 110 bodies with populations above 0.1m, 84 colonies with at least one automated mine and 276 with at least one civilian mining complex.
There are over 6000 ships in 2000 fleets. About 60% of the fleets are civilian. I added extra shipping lines to multi-system NPR (1 for every 2 transits), but I think I might remove that and just start them with a single line.
I also noticed a LOT more civilian mining colonies, because the NPRs often start with several systems above 10m pop so it is easier for them to spawn. I think that is fine, as it definitely creates the impression of a more developed Empire.
One minor error popped up, which I fixed. Otherwise, the run was error-free. At this point, I am going to setup a more serious campaign with a TN player race. Maybe Babylon 5, or maybe Enterprise era of Star Trek, both of which lend themselves to single-system player vs developed alien empires.