A report from Bloomberg explains that Apple's employees were notified by the company of a shortage of iPhone parts and replacements because of the industry-wide disruption caused by the corona virus. As per unidentified Apple Store employees, the company has warned Apple technicians that replacement iPhones (the ones that replace heavily damaged iPhones) will be low on supply in the next two to four weeks. In addition, Bloomberg reports that some individual Apple Stores are short of parts for repair. Wherever possible, Apple will replace individual parts, but for some situations...
Source: https://www.gsmarena.com/apple_stores_short_on_iphone_parts_due_to_coronavirus-news-41831.php