how limit the number of letters/words in excerpt?

Hey,

go to the Edit Post page and click “Options” in the very top right corner. Make sure the checkbox for “Excerpt” is checked.

After that, a new input field for the excerpt should appear at the bottom of the page. Type in a short sentence describing your post.

Thanks u..

But I need to make it manually for each and other post..
what to do in .php code.. any help ..

my blog is https://www.techover.in