How many countries in the world say “Tata” when you say bye. And how did that happen?

It’s almost a word when we were kids.

It is a word from when you were kids. It originated as a “nursery word”, as the OED puts it (i.e. baby talk), meaning both “good bye” and “walk”:

1823 S. Hutchinson Let. Sept.–Oct. (1954) 261 Baby I believe has not learnt any new words since Mrs M. wrote last, but she has the old ones very perfect—‘Gone’—‘Ta ta’—‘By bye’.

1886 J. Sully Teacher’s Handbk. Psychol. A child of eighteen months will mentally rehearse a series of experiences, as those of a walk: ‘Go tata, see geegee.’

In fact, it was used to name a theory of the origin of language as iconic gesture (which is presumably a baby-talk kind of thing):

ta-ta theory adj. Philology the theory that language originated in an attempt to imitate the body’s gestures with the vocal organs.