To determine n in cell culture experiments try to compare your setup to a mouse experiment.
Measuring 20 cells from the same culture is essentially identical to measuring 20 cells from the same mouse. Thus, the measurements are technical replicates: the cells come from the same culture, have grown in the same conditions. You can calculate a mean or a median but the measurements will not contribute to n.
When you grow and treat the cultures and then measure aliquots of cells in different devices (wells, chambers…), they are still technical replicates, it’s similar to measuring the same mouse in different microtiter plates.
Only when you grow, treat and measure the aliquots of cells separately, you can analyze them as biological replicates.
Excellent publication on how to define n in cell culture and animal experiments.