You probably will not see "massive cell death", but you will lose a fair amount.  I would plate 2x more than I needed the next day.  In my hands 12-well or 24-well plates always helped seemed to provide optimal area/volume.  I have done un-stimulated (no IL2) for 24h and was able to do RNA/protein extraction yielding decent quality.  Didn't calculate viability by flow. 

Mattew is right, you can keep the NK without il2, you can use 10% human serum to have better survival.

will 10IU/ml of IL2 activate NK?

Not sure we use 400iu, i know you can much less. You need to be careful cause low concentration of il2 boosts cd56 bright nk cells, so it moght skew your results

as i know 400IU is sufficient to activate NK cells. However, I do not want to activate them. I am wondering whether 10IU/ml is sufficient to sustain their viability without activate them

using low dose 10iu may lead to expansion of cd56 bright cells and distort your results.

i think it is better not adding any il-2 if you want the "fresh" phenotype

i see. Thank you very much Pinchas.