Less vs. Fewer for validation message?

Should an inputs validation message read as A or B for each of the following?

A Nick Name must be 10 characters or less.

B Nick Name must be 10 characters or fewer.

---

A Number must be 10 digits or less.

B Number must be 10 digits or fewer.

---

A Name must be 10 letters or less.

B Name must be 10 letters or fewer.