Blog excerpt

I’ll try out this theme in localhost before real use of it. I have made some “dummy” blogs and noticed that the READ MORE button shows after around 57 words. Is there any way to change this so it shows after 20 words or less?

I’ve tried the thing on https://en.support.wordpress.com/more-tag/ but I don’t get any link that says “Keep on reading!” I see only the text before this tag when it’s excerpted. Who can I do this visible?