Get the url of the portfolio page if the number of items to be shown is lower than the total number of items.

portfolio-page-url.php

if ( ! function_exists( 'genius_portfolio_url' ) ) :
/**
 * Echo a read more url for the portfolio page if the total posts is higher dan the displayed posts.
 */
function genius_portfolio_url() {
	// Return if the post limit is more than total posts.
	$post_limit = get_theme_mod( 'genius_portfolio_limit', 9 );
	$total_posts = wp_count_posts( 'jetpack-portfolio' )->publish;

	if ( $post_limit > $total_posts ) {
		return;
	}

	// Get the portfolio page url.
	$args = [
		'post_type'		=> 'page',
		'fields'		=> 'ids',
		'nopaging'		=> true,
		'meta_key'		=> '_wp_page_template',
		'meta_value'	=> 'template-parts/template-portfolio.php'
	];
	$pages = get_posts( $args );

	foreach ( $pages as $page ) {
		$portfolio_id = $page;
	}

	$portfolio_url = get_page_link( $portfolio_id );

	// Get the read more text.
	$read_more = get_theme_mod( 'genius_portfolio_read_more', __( 'Discover more' ) );

	echo '<a href="' . $portfolio_url . '">' . $read_more . '</a>';
}
endif; // genius_portfolio_url