So these food objects could be plural or singular depending on context?
Yes, that's right.
It depends on the context.
I understand that when "sebuah" (one/a/an) is written it eliminates the ambiguity...
Yes, that's also correct.
When "sebuah" (or another counter/classifier) is used, then it should be obvious if it's singular/plural.
For example :
Kamu makan sebuah apel = You eat an apple.
Kamu makan dua apel = You eat two apples.
Kamu makan banyak apel = You eat many apples.