When we talk about nonce in Initialization Vector(IV), is it required to used numbers only? not any letters or special characters?
No. In the context of the Initialization Vector of a cipher operated in some mode, the most significant property of a cryptographic nonce is that it is not reused. It often matters that it is random and unpredictable (e.g. with CBC). It is immaterial that it represents a number or anything else under some particular encoding.
If an IV is chosen at random (which is standard, and best), it would be bad to restrict it to represent a number (say, restricting each byte to be a digit according to ASCII), in particular because it would dramatically increases the odds of reusing the same IV.