The only acceptable number, but not only in Europe, everywhere, is “0”.
And if you already don’t know why: a billionaire has way too much money and power, and can influence governments in their favor, creating unsurmountable inequality and subverting the system’s rules. Billionaires are the cause why capitalism is failing (and since no one can stop them now, it will completely fail, and will drag the environment, and human survival, with it).