Apple warned its technical staff at the retail stores about the shortage of replacement iPhones. According to Apple, the shortage will be for the next two or four weeks.
The shortage of supply is only for the replacement iPhones for heavily damaged devices. Apple told its employees that they could mail the replacement iPhones to its customers and also to provide loaner devices, which will ease the time to wait.

However, there have been no details as to why this shortage will occur. Apple did not share the details and the reason for the short supply of these devices with its employees. Apple might be supply constraints because of a delay in production, and that delay in production is because of coronavirus outbreak. Many Apple employees at retail stores told the news that a few stores have a shortage of individual components used for device repairs.
The major Apple suppliers, such as Foxconn and Pegatron, shut down at the high production time during February, is now causing supply issues. The factories are now back and running, but the production shortage is not at normal levels as there are still many employees in those factories who are not working due to coronavirus. Also, Apple imposing travel restrictions play a part in low production.
Foxconn, which is Apple’s main iPhone assembler, is not functioning at 100%. It will resume its normal operation by the end of March 2020. It is why there has been a shortage of replacement phones, which will continue for the next two weeks.