I expect that by 1975, say, the number of high-end hardware installations was a lot more than 80. Multics, even when completed, didn't make much of a dent in the world. The ideas were influential, used in a number of later OSes, but Multics itself didn't go very far.
> Yes. It was also less capable. Both of those things were design goals of Unix and not of Multics.
Yes. Multics had the wrong design goals. Arguably, it hit them, but that didn't matter, because they were the wrong goals (though not obviously wrong at the time). That's why Multics is given short shrift - it successfully implemented what turned out in retrospect to be the wrong thing.
Why were they the wrong goals? There were something like 600,000 PDP-11s sold. Sure, Unix didn't go on all of those, and Multics could have run on more than the 80 machines that it did. Still, that's an insanely large number of installations that they... not exactly ignored, because even the first PDP-11 hadn't shipped when the Multics project began, but at least a huge number of machines that they were unable to serve. And of course, as always happens in this business, the lower-end machines became more and more powerful. The result of that (plus easier portability) was that Unix ate the world, and Multics became a footnote.