How many utopias have failed in the last 100 years?

The term "utopia" refers to an idealized world or society that is often used to critique or envision alternatives to current social systems. However, since utopias are by definition idealized and speculative, it is not possible to say how many have "failed" since they do not physically exist.

Could you clarify what aspect of utopias or social experiments you would like to know more about?