Looking again at stuff with some evidence but not clearly worked out in the original series:
In Foundation series, in Dead Hand we learn:
Bel Riose said:
these men of the Foundation swarm like senseless bees and fight like madmen. Every planet is defended viciously, and once taken, every planet heaves so with rebellion it is as much trouble to hold as to conquer. But they are taken, and they are held.
We later learn that Imperial forces captured "outer planets of Loris" and planet Loris itself. Plus a number of other unspecified planets.
The result as stated in the Mule:
Han Pritcher said:
But this history you mention became inevitable only after we had fought desperately for over a year. The inevitable victory we won cost us half a thousand ships and half a million men.
Really?
The combined population of multiple planets of Loris kingdom, or multiple planets captured in Loris and elsewhere is in the magnitude of over 10 000 millions. With the situation like "fight like madmen. Every planet is defended viciously, and once taken, every planet heaves so with rebellion it is as much trouble to hold as to conquer. But they are taken, and they are held.", "500 000 men" is unrealistically small tally of losses. A realistic number would be something like 500 million men, women and children.
Is it just a case of Asimov cannot do mathematics? Plenty of examples of that...
And yet, in this case, there is an alternative explanation.
For planet Terminus itself, which had had mere 5 million households with TVs 45 years earlier, total population of 50 million or so in 200 FE and 500 000 casualties, all men, in a desperate war fought at a distance, in outer space and other planets, is a sensible order of magnitude.
So the logical explanation is that the "us" whose losses Pritcher is counting is just the men of Terminus. The men, women and children from other planets who died for Foundation were not "us" and did not count.
And remind who Han Pritcher was:
Indbur III said:
I have your record here, captain — complete. You are forty-three and have been an Officer of the Armed Forces for seventeen years. You were born in Loris, of Anacreonian parents, no serious childhood diseases, an attack of myo . . . well that's of no importance . . . education, pre-military, at the Academy of Sciences, major, hyper-engines, academic standing . . . hm-m-m, very good, you are to be congratulated . . . entered the Army as Under-Officer on the one hundred second day of the 293rd year of the Foundation Era.
Han Pritcher was himself born of the Four Kingdoms parents, yet he got to move to Terminus. And
he is sharing the attitude - that it is the Terminus people who counted, and his ancestors did not. Suggesting that this is pervasive attitude of Foundation, both for those born there and for those with luck to be able to immigrate.