B2 visitor visa extension for 2nd time

I had applied for My mother’s B2 visa extension (filed I539 online) in Apr 2020 due to the covid situation. However the status is still showing as received. The original i94 expired in May 2020. How long can my mother stay in US in this situation ? do I need to apply for another extension if the decision is not received by Dec ?
if I get the approval by then and I need to extend for 2nd time, will there be any challenges ? Experts Please suggest/help.