How common in sudden death in athletic people?

Varies. It's impossible to give a straight forward answer to this question due to the amount of confounding factors that could be involved. However, the most common cause of sudden death in athletes is a condition called hypertrophic cardiomyopathy, which is relatively rare and is estimated to cause death in less than 100 competitive athletes in the US every year.
Less than in others. Death on the playing field raises the suspicion of hypertrophic cardiomyopathy, electrolyte disturbance, or Cocaine abuse. Sudden death is most often due to coronary artery atherosclerosis )ascvd) and this is much less common in athletic folks, both because exercise protects and because athletes have other healthy habits. Yet ascvd's still the most common cause of death even in fit middle-agers.