Was there any research on what order of magnitude a and b have? I guess b must be in the order of dozens of thousands, since we are talking about people. What is a? Is it dominated by the number of species of insects, or cosmic bodies, or what?

but what would be the unit of measurement? Also, per analogiam: new cameras' resolution is improving from year to year. When exactly should it stop?  There's no easy answer, because all depends on how much you think you should be able to magnify a picture without pixelization. 

I'd say that for all practical purposes Wikipedia will be saturated when the vast majority of searches is covered, and users find abundance of information for whatever topic they research, and this information is given to them at the exact level of sophistication they're ready to comprehend. We're waaay far from this ideal.