Typically, you only want to declare something as a NUMBER if you are going to use it to perform math on the field. This is a way of sending a message to programmers that this field is essentially immutable for this record, at least without good reason.

Second, who says that an SSN needs to be a number? By declaring it as alphanumeric, you would have much less chance for pain if all the Social Security numbers are used up. We would have a second Y2K if the government ever decided to switch, but it would be more likely that they would introduce characters to the SSN rather than try to enlarge the record.