Since wealth is at least in part relative, there's some limit for how many people can be at the very top in tech as well. If millions of people had as much money as Sergey Brin has, then having $15b dollars wouldn't mean as much, or buy him as much. One way of thinking of it is how many people you could hire to work for you, which depends on the ratio between your wealth and their salaries.
But don't confuse money (which can be inflated an has no inherent unit of conversion to the material world or labor) with material wealth. If everyone had, say, a Star Trek 3-D printer and power/atom supply, we'd all be pretty well off.
I agree they're different, but I think something like labor-wealth is what a lot of people aspire to when they say they want to be wealthy. They don't just want to have a lot of stuff, but want to not have to work anymore to have it (i.e. own 100% of their own time), and they often want to be able to hire significant amounts of services from others (hire a personal chef, stay at fancy full-staff resorts, have a personal trainer, etc.). At least for now; perhaps as high-level robotics advances the wealthy will no longer be as interested in hiring human services.
I think it'll remain true at the higher levels of wealth, though, because if you have a lot of wealth, what it really translates into isn't just material goods (there's diminishing returns on how many of those you really need past a few hundred million $s), but the power to significantly change what a non-trivial number of people work on. You can start a company and hire 5,000 people to work on a project you think should be worked on; you can take a different route by initiating a prize like the X-Prize to encourage people to work on it that way; you can start a foundation or grant program; you can singlehandedly decree that [movie genre] is going to be a significant new segment, by promising to plow $1 billion into it; etc. The main thing huge sums of money buy you, imo, is this relative power in the economy, to push people to work on what you think should be worked on--- which necessarily requires that your wealth has to be large in relative terms, as a fraction of the economy.
What limits the number of spots at the top is the total amount of resources allocated there. If the über-rich got less über, there would be more free spots there. Plus, if the gap between the very rich and the very poor is smaller, with a fairly large middle class in between, a lot of social tensions disappear.
