apples is also accepted. The plural marker 들 is usually dropped after inanimate nouns. Note that after 이, 그, or 저, the plural marker cannot be dropped.
Technically this is singular. But in Korean in a lot of cases, you don't have to use plural markers. In fact in a lot of cases using singular sound more natural. Plurals are usually implied in the sentence structure.
In this case, it can mean both apple or apples. If clarification is needed, you can always ask "how many".