Make a threshold that if the relative wealth is equal between 100,000,000 million people no one is executed. That's a hard cap right there.

Problem is the rich would start making death contracts. Scooping up some poor person to put them at the top with a contract saying if they go through with it they get certain privileges. The person would have to be too stupid to realize they can just distribute the wealth given to them and its win win win.

Also would hide their money off shore.

The process of determining who is most wealthy would be problematic.

It would be difficult to have an accurate monthly system.