Question

I would to know how can I set the limit number of the posts visible into a page of a specify category.

Answer 1

Put this in your functions.php

function main_query_mods( $query ) {
    if(!$query->is_main_query()) {
        return;
    }
    // show 15 posts per page if category has id 7
    // check http://codex.wordpress.org/Conditional_Tags#A_Category_Page
    if ( is_category('7')) {
        $query->set('posts_per_page',15);
    }
}
add_action( 'pre_get_posts', 'main_query_mods' );