There are similarities in different words in languages. But the word for “2” is very similar in most of languages. Why this number is so special?

To build on Matthew McVeagh’s answer and comment:

Go to the renowned Zompist Numbers List.

Two and Three, *duwō and *treyes, are reasonably similar across Indo-European.

One gets conflated with Single/Same, *oynos / *sem, and ends up looking different.

Four and Five have a *kw, which went different ways in different languages, and get affected by analogy: *kwetwores *penkwe

More analogy kicks in with higher numbers, and they are less frequent to begin with as Matthew pointed out. (We have a long-standing suspicion that 7 is a loan from Akkadian, and 10 is somehow related to the word for fingers.) (EDIT: “two hands”.) But they’re still surprisingly consistent.