When we talk about a Number used ONCE (NONCE) in Initialization Vector (IV), is it required to use numbers only? Is is possible to use 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, 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.