Yes, MAX20801 can work with less than 20 solar cells in series. In this case ensure that the voltage of the cell series comes within the input voltage range of the IC (from 6.5V to 15.5V).
MAX20801
Not Recommended for New Designs
The MAX20801 family of Cell-String Optimizers enables cell-string Maximum Power Point Tracking (MPPT), providing superior photovoltaic (PV) module energy...
Datasheet
MAX20801 on Analog.com
Yes, MAX20801 can work with less than 20 solar cells in series. In this case ensure that the voltage of the cell series comes within the input voltage range of the IC (from 6.5V to 15.5V).