Those are just round zeros. You could obtain that much money in some African countries over the last 50 years for a few dollars. The latest cryto coin could give you that many coins. But these are numbers trying to represent wealth.
Any cap you put on the top could have the effect of reducing the value of a currency meaning the buying power doesn't change. A billion cap would make your million dollar house worth $100. However you slice it you are poor and someone else is rich.
A much more reasonable proposal would simply be to ensure that billionaires actually pay the ~25%-35% everyone else pays and to put a $ cap on itemized deductions so that billionaires find it vanishingly difficult to write off 100% of their taxes. Same thing should go for corporations. Right now the standard for FANG-like companies is to achieve a $0 tax bill at the end of the year. It's disgusting when those same companies have employees making minimum wage as 1099 workers and paying a double digit percent of those wages in federal income tax.
The problem with billionaires though is they typically make and hold their money through assets, so what we really need is a bunch of reforms on how we tax assets. Namely, it should be such that if you have more than $1B in assets, taxes eat away at it pretty swiftly until it is back down to something reasonable. I think that's something we all can get behind.
Any cap you put on the top could have the effect of reducing the value of a currency meaning the buying power doesn't change. A billion cap would make your million dollar house worth $100. However you slice it you are poor and someone else is rich.