I rarely hear the word "fruits" to indicate the plural of fruit. A basket of fruit is understood to mean that it contains several individual units of fruit.
Well, sort of ... except that kupa basically has to have the object marked, the person/people who are receiving something. You could say tu
wape matunda "We should give them fruit", for example, but tu--pe, without an object, is more likely to (or has to?) be interpreted as --tupe, without a subject (meaning that the subject must be u-).
I also really disagree with the "have to" interpretation this course uses for the subjunctive, but that's another point.