JSFiddle - React, Tailwind, and code Playground

HTML

<input name="index_pochta" type="text" class="form-control" id="checkout-index_pochta" />
<div class="result"></div>

JavaScript

$("#checkout-index_pochta").keyup(({ target }) => {
  if (target.value.trim().length >= 6) {
      $('.result').text(target.value.trim().length)
  }
});