Why tax them? If they made their money fair and square, it's their's and their inheritors to keep.
Islam already solved this over 1400 year ago. Zakat laws are fair, and not egregious like some of the crazy taxation proposals we've been seeing thrown around, with no understanding how they will actually destroy the economy. Zakat has 0% income tax, 0% gains tax, 0% inheritance tax, and only taxes certain things at a very reasonable fixed rate (e.g. money that is 1 year old is "taxed" at 2.5% which is payable to charity among other things). This encourages people to work for wealth, and sustains the economy and society as has been proven historically.
A huge underlying issue that no one is willing to talk about is how the modern economic system is built on lending (with interest), a predatory and dangerous practice that has been prohibited thousands of year ago by Islam, Christianity, and Judaism. Fix the underlying problem (and anything that offshoots from it, such as stock shorting), and things will start to fall in place.
Much of this covers from options in the brands they built.
The stocks aren't being taken from anyone.
So... Why would there be a limit? How would you enforce it?