> How do we determine that limit ?

How about a cap relative to GDP?

Elon Musk's net worth is 3% of USA's annual GDP and 1% of global GDP.

I'm not a fan of limiting wealth among the upper class. But I am a fan of stopping a small number of individuals from controlling a significant percent of the world's GDP because if that trend continues, we'll end up with individual people more powerful than the governments that are supposed to keep them in check.

By legislation and policymaking - just like any other numbers we as a society end up picking (like the age to get a drivers license or buy alcohol or the income tax brackets).

Just like those cases, what other countries are doing would mostly be irrelevant - except, just like now, people may try to find arbitrage opportunities by getting creative about where they live.