So, I did an experiment. As you can see, I filled the carafe of the French Press with water. I think the photo editing makes it easier to see the waterline. And then I poured the water into measuring cups to see what 1.5 liters was in cups. Granted, I could have googled it, but that would have deprived us of these pictures...
I took out my classic Pyrex measuring cup, which may be a "museum piece" by now, and started pouring. I discovered that it is smaller than the French Press. I had to then draft the plastic measuring cup to handle the extra, the "overflow." To simplify, the 1.5 liter is at least five and a half cups, probably six, since I didn't fill to the maximum.
Then I took out my three giant mugs and poured the water from the measuring cups into the mugs, up to the level I'd have for coffee. I do like a bit of milk with my coffee. This confirmed my original estimate that for "just me" on a normal day, I need to fill the French Press no more than 2/3 full. Yes, I drink two giant mugs' worth of coffee.
And for those who drink "normal" sized coffee mugs, I then took out a variety of that size.
Yes, though if everyone drinking is of the "no milk, fill it to the top," I'd estimate five mugs full of coffee, "black, no milk."
Now I can be a better hostess.