Without context, any noun in Indonesian can be singular or plural. We only use word repetition if we want to emphasize that the noun is plural. In these example, we don't need to emphasize that the object is plural :

I don't eat apples -> saya tidak makan apel; Apples are healthy foods-> Apel adalah makanan sehat;

Meanwhile, in these examples, emphasizing that the object is plural is necessary, because we are referring to a specific group of objects : Those apples are delicious -> Apel-apel itu enak; These apples are green -> Apel-apel ini hijau;

Furthermore, we can also use a counter if we want to emphasize that an object is singular :

I eat an apple this morning -> Saya makan sebuah apel pagi ini;

Without the counter, the apple can be singular or plural, so I might have eaten 1 apple or 10.