Mysql – How to decide the maximal characters count for VARCHAR-like type columns in databases if specified value is actual only for one-byte characters?

In contrast to CHAR, VARCHAR values are stored as a 1-byte or 2-byte
length prefix plus data. The length prefix indicates the number of
bytes in the value. A column uses one length byte if values require no
more than 255 bytes, two length bytes if values may require more than
255 bytes.

MySQL official documentation

Let us consider this problem on EC store case.
Here is how could be defined the product (item) entity (most likely you can understand this code even has not learned the TypeScript):

import { FIXED_CHARACTERS_COUNT_IN_UNIVERSAL_UNIQUE_ID__VERSION_4 } from "fundamental-constants";


type Product = {
  readonly ID: Product.ID;
  label: string;
  price__dollars__withoutTaxes: number;
};


namespace Product {

  export type ID = string;
  export namespace ID {
    export const TYPE: StringConstructor = String;
    export const REQUIRED: boolean = true;
    export const FIXED_CHARACTERS_COUNT: number = FIXED_CHARACTERS_COUNT_IN_UNIVERSAL_UNIQUE_ID__VERSION_4;
  }

  export namespace Label {
    export const TYPE: StringConstructor = String;
    export const REQUIRED: boolean = true;
    export const MINIMAL_CHARACTERS_COUNT: number = 2;
    export const MAXIMAL_CHARACTERS_COUNT: number = 127;
  }

  export namespace Price__Dollars__WihtoutTaxes {
    export const TYPE: NumberConstructor = Number;
    export const REQUIRED: boolean = true;
    export const MINIMAL_VALUE: number = 0;
  }

}

Both inputted data validation on the frontend side and request data validation at backend same as database definition must obey to above business rules. Particularly, the product label must include from 2 to 127 characters:

Assume that above values are never directly inputted twice at both frontend and backend – instead, it is been referred:

<!-- BAD: the maximal characters count has been HARDCODED -->
<label for"PRODUCT_LABEL--INPUT">Please input 2-127 characters.</label>
<input type="text" maxlen="127" id="PRODUCT_LABEL--INPUT" />

<!-- GOOD: the maximal characters count has been referred (no matter what is the template engine)-->
<label for"PRODUCT_LABEL--INPUT">Please input {{ Product.Label.MINIMAL_CHARACTERS_COUNT }}-{{ Product.Label.MAXIMAL_CHARACTERS_COUNT }} characters.</label>
<input type="text" maxlen="{{ Product.Label.MAXIMAL_CHARACTERS_COUNT }}" id="PRODUCT_LABEL--INPUT" />

When defining the database (now matter, how exactly – via raw SQL request, GUI tool or ORM), we also will set the VARCHAR-like type for the label column of the Products tables with 127 characters maximal length (again, by referring to Product.Label.MAXIMAL_CHARACTERS_COUNT instead of direct input of 127 value).

Then, assume that the seller has inputted the product label consists almost 127 characters, but including 2-byte ones. Validation on the frontend has not been threat the inputted value same as validation of request data at the backend. But once the server application will try to save the added (or updated) product to the table, we’ll get the exception about label‘s value is exceeding the maximal length!

Question: which value must be set in Product.Label.MAXIMAL_CHARACTERS_COUNT?
(Let me repeat that this value is being referred from both frontend and the backend).