"You need to wash several cups, five or seven."
Translation:Тебе надо помыть несколько чашек, пять или семь.
тебе надо мыть несколько чашек, пять или семь should be correct also, no? Or why do we need perfective here? There is no context...
If it would have been a general rule, like "always wash several cups, five or seven", then imperfective could be right, but here it sounds like an one-time order, so I guess perfective is more on it's place.