get_number_of_posts_in_term.php

<?php
//Show the number of posts in an archive page

//get the term object
$term = get_queried_object();
//get the number of posts in that term
$term->count;


//Show the number of posts of any term by any identifier (e.g. for the term category and the term with id 1)

$term = get_term_by( 'id', 1, 'category' );
if ( ! empty( $term ) ) {
    echo $term->count;
}