How to limit the number of users who can be added to a user custom taxonomy?

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I have created a user custom taxonomy called institutions and when an administrator/NationalCouncil role is creating a user account, he or she should be able to allocate that user to one institution. However, I want to set limits on the institution terms i.e. if I have the term "Kyambogo University" I should be able to set a limit for the number of users who can be assigned to this term. When a limit is reached, an error should be displayed. How best can I achieve this? Here is the code below which I used to create a user custom taxonomy.

<? php

// Register Custom Taxonomy
function custom_taxonomy() {
  $labels = array(
    'name'                       => _x( 'Institutions', 'Institutions Name', 'user_taxonomy' ),
    'singular_name'              => _x( 'Institution', 'Institution Name', 'user_taxonomy' ),
    'menu_name'                  => __( 'Institutions', 'user_taxonomy' ),
    'all_items'                  => __( 'All Institutions', 'user_taxonomy' ),
    'parent_item'                => __( 'Parent Institution', 'user_taxonomy' ),
    'parent_item_colon'          => __( 'Parent Institution:', 'user_taxonomy' ),
    'new_item_name'              => __( 'New Institution Name', 'user_taxonomy' ),
    'add_new_item'               => __( 'Add Institution', 'user_taxonomy' ),
    'edit_item'                  => __( 'Edit Institution', 'user_taxonomy' ),
    'update_item'                => __( 'Update Institution', 'user_taxonomy' ),
    'view_item'                  => __( 'View Institution', 'user_taxonomy' ),
    'separate_items_with_commas' => __( 'Separate Institution with commas', 'user_taxonomy' ),
    'add_or_remove_items'        => __( 'Add or remove Institutions', 'user_taxonomy' ),
    'choose_from_most_used'      => __( 'Choose from the most used', 'user_taxonomy' ),
    'popular_items'              => __( 'Popular Institutions', 'user_taxonomy' ),
    'search_items'               => __( 'Search Institutions', 'user_taxonomy' ),
    'not_found'                  => __( 'Not Found', 'user_taxonomy' ),
    'no_terms'                   => __( 'No Institutions', 'user_taxonomy' ),
    'items_list'                 => __( 'Institutions list', 'user_taxonomy' ),
    'items_list_navigation'      => __( 'Institutions list navigation', 'user_taxonomy' ),
  );
  $args = array(
    'labels'                     => $labels,
    'hierarchical'               => true,
    'public'                     => true,
    'show_ui'                    => true,
    'show_admin_column'          => true,
    'show_in_nav_menus'          => true,
    'show_tagcloud'              => true,
  );
  register_taxonomy( 'Institutions', 'user', $args );
}
add_action( 'init', 'custom_taxonomy', 0 );


 /**
 * Admin page for the 'Institutions' taxonomy
 */
function cb_add_Institutions_taxonomy_admin_page() {

  $tax = get_taxonomy( 'Institutions' );

  add_users_page(
    esc_attr( $tax->labels->menu_name ),
    esc_attr( $tax->labels->menu_name ),
    $tax->cap->manage_terms,
    'edit-tags.php?taxonomy=' . $tax->name
  );

}
add_action( 'admin_menu', 'cb_add_Institutions_taxonomy_admin_page' );

/**
 * Unsets the 'posts' column and adds a 'users' column on the manage Institutions admin page.
 */
function cb_manage_Institutions_user_column( $columns ) {

  unset( $columns['posts'] );

  $columns['users'] = __( 'Users' );

  return $columns;
}
add_filter( 'manage_edit-Institutions_columns', 'cb_manage_Institutions_user_column' );

/**
 * @param string $display WP just passes an empty string here.
 * @param string $column The name of the custom column.
 * @param int $term_id The ID of the term being displayed in the table.
 */
function cb_manage_Institutions_column( $display, $column, $term_id ) {

  if ( 'users' === $column ) {
    $term = get_term( $term_id, 'Institutions' );
    echo $term->count;
  }
}
add_filter( 'manage_Institutions_custom_column', 'cb_manage_Institutions_column', 10, 3 );


/**
 * @param object $user The user object currently being edited.
 */
function cb_edit_user_Institution_section( $user ) {
  global $pagenow;

  $tax = get_taxonomy( 'Institutions' );

  /* Make sure the user can assign terms of the Institutions taxonomy before proceeding. */
  if ( !current_user_can( $tax->cap->assign_terms ) )
    return;

  /* Get the terms of the 'Institutions' taxonomy. */
  $terms = get_terms( 'Institutions', array( 'hide_empty' => false ) ); ?>

  <h3><?php _e( 'Institutions' ); ?></h3>

  <table class="form-table">

    <tr>
      <th><label for="Institutions"><?php _e( 'Allocated Institutions' ); ?></label></th>

      <td><?php

      /* If there are any Institutions terms, loop through them and display checkboxes. */
      if ( !empty( $terms ) ) {

        foreach ( $terms as $term ) { 
        ?>
          <label for="Institutions-<?php echo esc_attr( $term->slug ); ?>">
            <input type="checkbox" name="Institutions[]" id="Institutions-<?php echo esc_attr( $term->slug ); ?>" value="<?php echo $term->slug; ?>" <?php if ( $pagenow !== 'user-new.php' ) checked( true, is_object_in_term( $user->ID, 'Institutions', $term->slug ) ); ?>>
            <?php echo $term->name; ?>
          </label><br/>
        <?php
        }
      }

      /* If there are no Institutions terms, display a message. */
      else {
        _e( 'There are no Institutions available.' );
      }

      ?></td>
    </tr>

  </table>
<?php }

add_action( 'show_user_profile', 'cb_edit_user_Institution_section' );
add_action( 'edit_user_profile', 'cb_edit_user_Institution_section' );
add_action( 'user_new_form', 'cb_edit_user_Institution_section' );

/**
 * @param int $user_id The ID of the user to save the terms for.
 */
function cb_save_user_Institution_terms( $user_id ) {

  $tax = get_taxonomy( 'Institutions' );

  /* Make sure the current user can edit the user and assign terms before proceeding. */
  if ( !current_user_can( 'edit_user', $user_id ) && current_user_can( $tax->cap->assign_terms ) )
    return false;

  $term = $_POST['Institutions'];

  /* Sets the terms (we're just using a single term) for the user. */
  wp_set_object_terms( $user_id, $term, 'Institutions', false);

  clean_object_term_cache( $user_id, 'Institutions' );
}

add_action( 'personal_options_update', 'cb_save_user_Institution_terms' );
add_action( 'edit_user_profile_update', 'cb_save_user_Institution_terms' );
add_action( 'user_register', 'cb_save_user_Institution_terms' );

/**
 * @param string $username The username of the user before registration is complete.
 */
function cb_disable_Institutions_username( $username ) {

  if ( 'Institutions' === $username )
    $username = '';

  return $username;
}
add_filter( 'sanitize_user', 'cb_disable_Institutions_username' );

/**
 * Update parent file name to fix the selected menu issue
 */
function cb_change_parent_file($parent_file)
{
    global $submenu_file;

    if (
      isset($_GET['taxonomy']) && 
      $_GET['taxonomy'] == 'Institutions' &&
      $submenu_file == 'edit-tags.php?taxonomy=Institutions'
    ) 
    $parent_file = 'users.php';

    return $parent_file;
}
add_filter('parent_file', 'cb_change_parent_file');


?>

That is a sample screenshot of the user custom taxonomy and that is where I wish to add limits which an admin can set a limit of how many users can be attached to the custom taxonomy term.

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