Cell numbers decides the voltage while the half or full cell decides the current rating of a system. For example, if a full cell has current of Isc and voltage of Voc then a half cell will have a current specification of 0.5*Isc and same voltage specification of Voc. MAX20801 was designed for 20~24 cells and if you cut 20 full cells to 20+20 half cells then we will need 2 x MAX20801 as shown below:
Thus, if you cut 20 full cells to 20+20 half cells then you will need 2 x MAX20801 where each MAX20801 still only support 20 half cells. This will also have no issues from the control algorithm point of view .