twin: one of two children born at the same time to the same mother
Ex: She's expecting twins.
The twins are now eight months old.
Ok, let say a woman has twins at her first birth (ie, there are only 2 children)
So, we say "I have 2 twins" or "I have twins"?
And , what if she has twins at her second birth.
Do we say "2 pairs of twins"?
"I have two twins/a set(pair) of twins" isn't ungrammatical, but it's more common and idiomatic to say "I have twins".
As for two sets of twins or two pairs of twins, both are correct. The former is a little more common.
"I have twins" is normal. "I have 2 twins" is redundant, and not used. If a mother is lucky (!) enough to have twins again at her second pregnancy, you would say "I have two pairs of twins" (although you would probably be too tired to engage in conversation). You can also use "a set of twins", which generalises to "a set of triplets" etc.
In old English the use was different. A twin (or getwinnas) meant two persons. An old English speaker would understand 2 twins to mean 4 people. This is not modern use.
I have twins - or - I have a set of twins.
Also related to the discussion, there is a major difference between "pair of twins" and "set of twins". Pair indicates a quantity of 2 on its own, whereas set has no quantity until the rest of the context identifies it. A Set of twins will always be interpreted properly as 2 people who are twins. A Pairs of Twins is easily misinterpreted as 4 people - 2 people in one set of twins and 2 people in the other set of twins, then paired together in a setting to make up a pair of twins. To reinforce it further, no one would say "I have a trio of triplets". However they might say "I have a set of triplets". But most common would be just "I have triplets".
