How much of a percentage can a person or corporation own in a certain industry before it becomes illegal?

The percentage of ownership that triggers legal restrictions varies by country and industry. In the U.S., for example, antitrust laws may limit ownership to prevent monopolistic practices, often scrutinizing acquisitions where a company would acquire more than 50% of a competitor. Additionally, certain industries like telecommunications and banking have specific regulations that cap ownership at lower percentages, sometimes around 25-30%. It is essential to consult relevant laws and regulations specific to the jurisdiction and industry in question.