functions.php

function rentfetch_available_units_label( $number ) {

	$number = intval( $number );

	if ( 0 === $number || empty( $number ) ) {
		$available = 'No units available';
	} elseif ( 1 === $number ) {
		$available = '1 unit available';
	} else {
		$available = $number . ' units available';
	}

	return $available;
}
add_filter( 'rentfetch_get_available_units_label', 'rentfetch_available_units_label' );