Barring unforseen early demises, the only vacancies Biden may get would be from the existing liberals. There are many who want to increase the number of justices so that Biden can have appointments to rectify what happened with Garland and then hypocritically McConnell took the opposite approach when Barrett was nominated. For the increase to happen, the dems would need to retake the House, Biden be reelected, the dems keep the Senate AND be willing to get rid of the fillibuster.