How much horsepower does a turbo add?

Could you elaborate on the question of "How much horsepower does a turbo add?" when considering the impact of a turbocharger on a vehicle's engine? I'm particularly interested in understanding the range of potential gains, as well as any factors that may influence the amount of horsepower added. Additionally, are there any common misconceptions or assumptions about turbocharged engines that you could clarify? A thorough explanation would be greatly appreciated as I'm seeking to gain a deeper understanding of this subject.