Add this code in functions.php (replace “20” with the desired number of words):

/* Set the maximum number of words in a post excerpt */
function mytheme_custom_excerpt_length( $length ) {
return 20;
}
add_filter( 'excerpt_length', 'mytheme_custom_excerpt_length', 999 );

Source: https://developer.wordpress.org/reference/hooks/excerpt_length/