How to set validation for a cell on how many characters inputted?

You can use a regular expression in the validation tab for your field to validate the input.

The regular expression

^.{6,7}$

validates that the field has a minimum of 6 characters and a maximum of 7.

Other than changing the set of numbers, you can also set it to a maximum, such as ^.{,7}$ (a maximum of 7 characters), or a minimum, ^.{6,}$ (a minimum of 6 characters).

For other kinds of validation, you can check here.