Spoilers!
Isaac Asimov said:
The council chamber was full in a very literal sense on the fourth day of the trial of Hober Mallow, Master Trader. The only councilman absent was feebly cursing the fractured skull that had bedridden him. The galleries were filled to the aisleways and ceilings with those few of the crowd who by influence, wealth, or sheer diabolic perseverance had managed to get in. The rest filled the square outside, in swarming knots about the open-air trimensional 'visors.
Note: at least the open-air visors were "trimensional"
Note: people still wanted to get into the actual chamber rather than settle for the trimensional visors.
Isaac Asimov said:
A lone beam of light centered upon him and in the public 'visors of the city, as well as on the myriads of private 'visors in almost every home of the Foundation's planets, the lonely giant figure of a man stared out defiantly.
Note: Foundation's planets. Which ones? Besides Terminus?
Note: "almost every" home. There were few homes in Foundation that lacked private visors. See later, checking the significance of numbers:
Isaac Asimov said:
Hober Mallow waited to let the significance sink in and was rewarded with the first silence he had yet encountered, as the gallery caught its collective breath. That was for the inhabitants of Terminus itself. The men of the Outer Planets could hear only censored versions that would suit the requirements of religion.
Which means that the men of the "Outer Planets" could not have watched the broadcast. Then which were the planets, plural, where myriads of private visors did show the trial? Just a few sentences back, we hear of multiple planets watching; now we see Terminus alone asserted. Flagrant contradiction in a few lines.
Isaac Asimov said:
In five million homes on Terminus, excited observers crowded their receiving sets more closely
So, that´s the number.
Note: observers crowded the receiving sets more closely.
Therefore:
the sets were not so large as to make crowding closer pointless;
the observers commonly crowded close to a single receiving set at home, rather than have multiple receiving sets at one home, with each resident watching his or her set
5 million is close to the total number of homes on Terminus, mostly having several residents. Homes that did not have any sets must have been rare; homes which were unoccupied at the time because everyone was out of home watching a public visor (or, as in case of Publis Manlio, expressly a bachelor, present at trial) may have been a minor contributor. We here have an estimate for the population of Terminus around 155 FE.