I'm afraid I don't understand the question. Are you asking about any specific time?

At any given time in any given kingdom, there were tens of thousands of nobles. A handful - those closely related to the ruling dynasty - could be heirs. Two examples spring to mind. Stephen of Blois succeeded his uncle, Henry I of England, after Henry's only son died in a shipwreck. When the last Capetian king of France, Charles IV, died without issue, the crown passed to a cousin, Philip of Valois, who had previously been count of Anjou, Maine, and Valois.