On the Guest module, go to the form and edit the fields. Click on the "Name" field and click Edit. Go down to Form Validation Properties and select a validator as "IsRegexMatch" and in the Validator Argument field, enter:
^[A-Za-z]{1,20}$
And that should allow only upper and lower case letters along with a minimum of 1 character and a maximum of 20 characters.
Might want to fact-check me on the [a-Z] part and make sure it includes both uppercase and lowercase, but that should effectively do what you need.