在存储时，占用字节数应该是根据数据的长度动态变化的，可以使用DUMP函数来查看NUMBER类型数据的内部存储格式。貌似除NULL外，最短1字节，最长21字节，你测试下看看。

DECLARE
  V_NUM NUMBER;
BEGIN
  V_NUM := 0;
  PRINT(DUMP(V_NUM,16)||'  ---'||V_NUM);

  V_NUM := 3;
  PRINT(DUMP(V_NUM,16)||'  ---'||V_NUM);
  
  V_NUM := 3.14;
  PRINT(DUMP(V_NUM,16)||'  ---'||V_NUM);
         --         1         2         3         4
         --1234567890123456789012345678901234567890
  V_NUM := 9876543210987654321098765432109876543210;
  PRINT(DUMP(V_NUM,16)||'  ---'||V_NUM); 

  V_NUM := 987654321098765432109.8765432109876543219;
  PRINT(DUMP(V_NUM,16)||'  ---'||V_NUM); 
END;