If Biden dies or is otherwise removed from office after Jan. 20 - i.e., after having held the office for a full two years - then his successor could theoretically complete the remainder of his term, and then still go on to be elected president twice.
If, however, Harris were to succeed him prior to Jan. 20, then she could finish his term - but could be elected only once.
Regards,
Ah, thanks for that explanation. I had forgotten about that timeframe.