My amended return shows delivered on March 19th but when I check my amended return status it shows nothing. How do I know they've received it?

What is it that shows that your amended return was delivered? Did you mail it with a service that provides tracking? If so, that's how you know that the IRS received it.

It normally takes the IRS up to 16 weeks to process an amended return. However, the IRS is currently not processing tax returns that were filed by mail, including amended returns, because of the Covid-19 pandemic. When they start to process them again they will have reduced staff and a huge backlog, so it will be a long time before your amended return is processed, or even shows up in the status on the IRS web site.