There's by no means a legal consensus about what constitutional remedies there are, if any, to a sitting President's ability to pardon. About the only remedy is political - impeachment. And Trump probably would get impeached for it. But the pardons would stay.
https://www.brookings.edu/opinions/...ers-feel-betrayed-by-his-pardon-of-marc-rich/
https://www.npr.org/2016/11/01/5002...es-files-on-bill-clintons-pardon-of-marc-rich
https://www.nbcnews.com/think/opini...ueller-targets-can-anyone-stop-him-ncna815861
"If Trump Tries to Pardon Mueller Targets, Can Anyone Stop Him?
Legally, Trump can pardon pretty much anyone he wants to for federal crimes."
https://www.brookings.edu/opinions/...ers-feel-betrayed-by-his-pardon-of-marc-rich/
https://www.npr.org/2016/11/01/5002...es-files-on-bill-clintons-pardon-of-marc-rich
https://www.nbcnews.com/think/opini...ueller-targets-can-anyone-stop-him-ncna815861
"If Trump Tries to Pardon Mueller Targets, Can Anyone Stop Him?
Legally, Trump can pardon pretty much anyone he wants to for federal crimes."