In this case here is little code and that will help!! You will have to use the code in functions.php file
/* Changed excerpt length to 200 words*/
function your_code_excerpt_length( $length ) {
return 20;
}
add_filter( ‘excerpt_length’, ‘your_code_excerpt_length’, 200 );
If you are not ok with the limit of 200, you can change to to any integer number you like.