<?php 
$excerpt = get_the_excerpt();
echo wp_trim_words($excerpt, 20, '...');
?>

Custom PHP code:

<?php 
function text_words_limit( $text, $limit = 30 ) {
	$text = strip_tags( $text );
	$words_array = explode( ' ', $text, $limit );
	if( count($words_array) >= $limit ) {
		array_pop($words_array); // remove last element because it contain the rest of the text
	}
	$text_limitted = implode( ' ', $words_array );
	if(strlen($text) > $text_limitted) {
		$text_limitted .= '...';
	}
	return $text_limitted;
}
echo text_words_limit( $content, 30 );
?>

• wp_trim_words()