_n( string $single, string $plural, int $number, string $domain = 'default' )

function _n( $single, $plural, $number, $domain = 'default' ) {
	$translations = get_translations_for_domain( $domain );
	$translation  = $translations->translate_plural( $single, $plural, $number );

	/**
	 * Filters the singular or plural form of a string.
	 *
	 * @since WP-2.2.0
	 *
	 * @param string $translation Translated text.
	 * @param string $single      The text to be used if the number is singular.
	 * @param string $plural      The text to be used if the number is plural.
	 * @param string $number      The number to compare against to use either the singular or plural form.
	 * @param string $domain      Text domain. Unique identifier for retrieving translated strings.
	 */
	return apply_filters( 'ngettext', $translation, $single, $plural, $number, $domain );
}