grammatical number - If twins are always plural, how do I refer to a single twin, or how do we count twins?

I've learned that twins in English are always plural, e.g. there are the twins. Then how do we count twins? If there are four people coming, those are two twins? There are two twins? What if there are two people coming, who are twins. There are one twins? Or still There is one twin?

My misunderstanding stems from the fact that the Dutch word "tweeling" (singular) refers to a single pair of twins (two people), and "tweelingen" (plural) would imply at least two sets of twins.

Answer

You say there is one twin. Twins, when referred to as a set, require an s, just like most other groups of objects.

There are the twins.
There is a twin.
There are the cats.
There is a cat.

And so on.

In the case of the four twins, you could say this:

Here come the four twins.

If you wanted to be more precise, you could say this:

Here come [the] two sets of twins.

Saying there are the twins does not imply any specific number, although many people would assume that there are two, since twins come in sets of two.