Biden can’t just expand the size of the court by himself. Congress has to give him the authority to do so.
The House of Representatives is controlled by Republicans, so any court expansion plan is dead in the water. Even if the Democrats controlled the House, they only have 51 (out of 100) seats in the Senate. There are two Democratic members of the Senate who are basically Republican-lite (Manchin of WV and Sinema of AZ, who is technically an independent). Those two would kill any bill that allowed any sort of progressive change.
If you don’t like it, then you need to do everything you can to fight for Democratic majorities in both chambers of Congress.