How add restriction of length and if field value if not a number in Siebel

You can use below eScript on Pre Set Field Value (recommended) or Set Field Value on Business Component Server Script.

if (FieldName = "SSN") 
{
var SSNNum = this.GetFieldValue("SSN");
var sErrTxt = TheApplication().LookupMessage("User Defined Errors","Wrong SSN");
var result = isNaN(SSNNum);  // is not a number
var length = SSNNum.length;  // Length Count

if (result)
{
TheApplication().RaiseErrorText(sErrTxt);
}
else if (length < "15" || length > "15") 
{
TheApplication().RaiseErrorText(sErrTxt);
}

}