[crazygringo]

Practically speaking, for maximum lengths, you generally want to limit code points or bytes, not characters. You don't want to allow some ZALGO monstrosity in a password that is 5 characters but 500 bytes.

For exact lengths, you often have a restricted character set (like for phone numbers) and can validate both characters and length with a regex. Or the length in bytes works for 0–9.

Unless you're involved in text layout, you actually usually don't wind up needing the exact length in characters of arbitrary UTF-8 text.