Definitely there is an upper bound, but it isn't the first step upgrade on Macs
. Apple have always been stingy with the baseline memory amount. It's normally half what I'd put in a machine I'd build myself to be comfortable with for the types of usage I'd put the machine to.
My general rule of thumb for apple products which has served me well so far is take the standard base model amount of memory and double it as a baseline.
That will give you breathing room and generally improve performance.
Anything beyond that (which isn't expensive in terms of total system purchase price) and you want to be more seriously evaluating your workload, but that first step - doubling the baseline is usually appropriate if you're anything more than a basic web user.
That first doubling usually gives you around 3x the actual usable RAM for applications and data, as the OS generally tends to consume around 1/4 to half of the baseline amount to run efficiently before you really do much with it. (e.g., 8 GB machine is typically using 4 GB for the OS plus basic web browsing, upping to 16 GB gives you 12 GB free after that instead of 4).
i.e., that first step up in memory upgrade is normally a pretty big bang for the buck.
edit:
128 GB for 1080p is definitely overkill, 64 GB is overkill. But that's far and away beyond what we're talking about here
That said, I ran 16 GB in my 15" 2011 MBP for a while (after the price came down from $1500 for 16 GB to a couple of hundred, lol) and even back then with macOS lion/mountain lion there was a noticeable improvement in performance between 8 and 16. The 4GB it shipped with was a joke.