if 8 moles of potassium superoxide reacts with 5 moles of water how many moles will remain underacted?

Simple stoichiometry ....

4KO2(s) + 2H2O(l) --> 4KOH(aq) + 3O2(g) ............ balanced equation

8 mol ....... 5 mol ............. initial

8 mol ....... 4 mol ............. reacts ..... mole ratio of KO2 to H2O is 2:1

0 mol ....... 1 mol ............. excess moles

1 mol of water will remain unreacted.

underacted ?? do you mean unreacted?

is this the reaction?

4KO₂ + 2H₂O → 4KOH + 3O₂

the reaction is 4 moles of KO₂ reacts with 2 moles of H₂O

in your case, 8 moles of KO₂ would fully react with 4 moles of H₂O leaving 1 mole of H₂O un-reacted.

(edited, corrected)

Moles don't react to anything. They are just digging.