I think it's clear that the following two sentences are grammatical:
Is that a cup I see?
Are those cups I see?
However, what if you seek to inquire as to how many cups you can see?
Is that one cup or two cups I see?
Are those one cup or two cups I see?
Which of the above two sentences is correct?
Best Answer
For the first pair of questions, we know how many things we are looking at, and are simply querying what the individual items are. For this reason, the question can, and must, reflect the number.
For the second pair of questions, we know that they are cups but not how many. The way we deal with this, when we don't know how many items, is to talk about it as the whole scene or ensemble, which is one item: this works regardless of whether there is one cup or two cups.
In this sentence, that is the scene or ensemble.